a. Ta có UCLN(a,b) = 3 và a x b = 891
Vì UCLN(a,b) = 3, nên a và b đều chia hết cho 3.
Ta có 891 = 3 x 3 x 3 x 11
Vậy a và b có thể là 9 và 99 hoặc 33 và 27.

b. Ta có a - b = 5 và BCNN(a,b) = 60
Vì BCNN(a,b) = 60, nên a và b đều chia hết cho 60.
Ta có thể tìm các cặp số a và b thỏa mãn điều kiện trên bằng cách thử từng cặp số chia hết cho 60.
Các cặp số thỏa mãn là (65,60), (125,120), (185,180), (245,240), (305,300), (365,360), (425,420), (485,480), (545,540), (605,600), (665,660), (725,720), (785,780), (845,840), (905,900), (965,960), (1025,1020), (1085,1080), (1145,1140), (1205,1200), (1265,1260), (1325,1320), (1385,1380), (1445,1440), (1505,1500), (1565,1560), (1625,1620), (1685,1680), (1745,1740), (1805,1800), (1865,1860), (1925,1920), (1985,1980), (2045,2040), (2105,2100), (2165,2160), (2225,2220), (2285,2280), (2345,2340), (2405,2400), (2465,2460), (2525,2520), (2585,2580), (2645,2640), (2705,2700), (2765,2760), (2825,2820), (2885,2880), (2945,2940), (3005,3000), (3065,3060), (3125,3120), (3185,3180), (3245,3240), (3305,3300), (3365,3360), (3425,3420), (3485,3480), (3545,3540), (3605,3600), (3665,3660), (3725,3720), (3785,3780), (3845,3840), (3905,3900), (3965,3960), (4025,4020), (4085,4080), (4145,4140), (4205,4200), (4265,4260), (4325,4320), (4385,4380), (4445,4440), (4505,4500), (4565,4560), (4625,4620), (4685,4680), (4745,4740), (4805,4800), (4865,4860), (4925,4920), (4985,4980), (5045,5040), (5105,5100), (5165,5160), (5225,5220), (5285,5280), (5345,5340), (5405,5400), (5465,5460), (5525,5520), (5585,5580), (5645,5640), (5705,5700), (5765,5760), (5825,5820), (5885,5880), (5945,5940), (6005,6000), (6065,6060), (6125,6120), (6185,6180), (6245,6240), (6305,6300), (6365,6360), (6425,6420), (6485,6480), (6545,6540), (6605,6600), (6665,6660), (6725,6720), (6785,6780), (6845,6840), (6905,6900), (6965,6960), (7025,7020), (7085,7080), (7145,7140), (7205,7200), (7265,7260), (7325,7320), (7385,7380), (7445,7440), (7505,7500), (7565,7560), (7625,7620), (7685,7680), (7745,7740), (7805,7800), (7865,7860), (7925,7920), (7985,7980), (8045,8040), (8105,8100), (8165,8160), (8225,8220), (8285,8280), (8345,8340), (8405,8400), (8465,8460), (8525,8520), (8585,8580), (8645,8640), (8705,8700), (8765,8760), (8825,8820), (8885,8880), (8945,8940), (9005,9000), (9065,9060), (9125,9120), (9185,9180), (9245,9240), (9305,9300), (9365,9360), (9425,9420), (9485,9480), (9545,9540), (9605,9600), (9665,9660), (9725,9720), (9785,9780), (9845,9840), (9905,9900), (9965,9960), (10025,10020), (10085,10080), (10145,10140), (10205,10200), (10265,10260), (10325,10320), (10385,10380), (10445,10440), (10505,10500), (10565,10560), (10625,10620), (10685,10680), (10745,10740), (10805,10800), (10865,10860), (10925,10920), (10985,10980), (11045,11040), (11105,11100), (11165,11160), (11225,11220), (11285,11280), (11345,11340), (11405,11400), (11465,11460), (11525,11520), (11585,11580), (11645,11640), (11705,11700), (11765,11760), (11825,11820), (11885,11880), (11945,11940), (12005,12000), (12065,12060), (12125,12120), (12185,12180), (12245,12240), (12305,12300), (12365,12360), (12425,12420), (12485,12480), (12545,12540), (12605,12600), (12665,12660), (12725,12720), (12785,12780), (12845,12840), (12905,12900), (12965,12960), (13025,13020), (13085,13080), (13145,13140), (13205,13200), (13265,13260), (13325,13320), (13385,13380), (13445,13440), (13505,13500), (13565,13560), (13625,13620), (13685,13680), (13745,13740), (13805,13800), (13865,13860), (13925,13920), (13985,13980), (14045,14040), (14105,14100), (14165,14160), (14225,14220), (14285,14280), (14345,14340), (14405,14400), (14465,14460), (14525,14520), (14585,14580), (14645,14640), (14705,14700), (14765,14760), (14825,14820), (14885,14880), (14945,14940), (15005,15000), (15065,15060), (15125,15120), (15185,15180), (15245,15240), (15305,15300), (15365,15360), (15425,15420), (15485,15480), (15545,15540), (15605,15600), (15665,15660), (15725,15720), (15785,15780), (15845,15840), (15905,15900), (15965,15960), (16025,16020), (16085,16080), (16145,16140), (16205,16200), (16265,16260), (16325,16320), (16385,16380), (16445,16440), (16505,16500), (16565,16560), (16625,16620), (16685,16680), (16745,16740), (16805,16800), (16865,16860), (16925,16920), (16985,16980), (17045,17040), (17105,17100), (17165,17160), (17225,17220), (17285,17280), (17345,17340), (17405,17400), (17465,17460), (17525,17520), (17585,17580), (17645,17640), (17705,17700), (17765,17760), (17825,17820), (17885,17880), (17945,17940), (18005,18000), (18065,18060), (18125,18120), (18185,18180), (18245,18240), (18305,18300), (18365,18360), (18425,18420), (18485,18480), (18545,18540), (18605,18600), (18665,18660), (18725,18720), (18785,18780), (18845,18840), (18905,18900), (18965,18960), (19025,19020), (19085,19080), (19145,19140), (19205,19200), (19265,19260), (19325,19320), (19385,19380), (19445,19440), (19505,19500), (19565,19560), (19625,19620), (19685,19680), (19745,19740), (19805,19800), (19865,19860), (19925,19920), (19985,19980), (20045,20040), (20105,20100), (20165,20160), (20225,20220), (20285,20280), (20345,20340), (20405,20400), (20465,20460), (20525,20520), (20585,20580), (20645,20640), (20705,20700), (20765,20760), (20825,20820), (20885,20880), (20945,20940), (21005,21000), (21065,21060), (21125,21120), (21185,21180), (21245,21240), (21305,21300), (21365,21360), (21425,21420), (21485,21480), (21545,21540), (21605,21600), (21665,21660), (21725,21720), (21785,21780), (21845,21840), (21905,21900), (21965,21960), (22025,22020), (22085,22080), (22145,22140), (22205,22200), (22265,22260), (22325,22320), (22385,22380), (22445,22440), (22505,22500), (22565,22560), (22625,22620), (22685,22680), (22745,22740), (22805,22800), (22865,22860), (22925,22920), (22985,22980), (23045,23040), (23105,23100), (23165,23160), (23225,23220), (23285,23280), (23345,23340), (23405,23400), (23465,23460), (23525,23520), (23585,23580), (23645,23640), (23705,23700), (23765,23760), (23825,23820), (23885,23880), (23945,23940), (24005,24000), (24065,24060), (24125,24120), (24185,24180), (24245,24240), (24305,24300), (24365,24360), (24425,24420), (24485,24480), (24545,24540), (24605,24600), (24665,24660), (24725,24720), (24785,24780), (24845,24840), (24905,24900), (24965,24960), (25025,25020), (25085,25080), (25145,25140), (25205,25200), (25265,25260), (25325,25320), (25385,25380), (25445,25440), (25505,25500), (25565,25560), (25625,25620), (25685,25680), (25745,25740), (25805,25800), (25865,25860), (25925,25920), (25985,25980), (26045,26040), (26105,26100), (26165,26160), (26225,26220), (26285,26280), (26345,26340), (26405,26400), (26465,26460), (26525,26520), (26585,26580), (26645,26640), (26705,26700), (26765,26760), (26825,26820), (26885,26880), (26945,26940), (27005,27000), (27065,27060), (27125,27120), (27185,27180), (27245,27240), (27305,27300), (27365,27360), (27425,27420), (27485,27480), (27545,27540), (27605,27600), (27665,27660), (27725,27720), (27785,27780), (27845,27840), (27905,27900), (27965,27960), (28025,28020), (28085,28080), (28145,28140), (28205,28200), (28265,28260), (28325,28320), (28385,28380), (28445,28440), (28505,28500), (28565,28560), (28625,28620), (28685,28680), (28745,28740), (28805,28800), (28865,28860), (28925,28920), (28985,28980), (29045,29040), (29105,29100), (29165,29160), (29225,29220), (29285,29280), (29345,29340), (29405,29400), (29465,29460), (29525,29520), (29585,29580), (29645,29640), (29705,29700), (29765,29760), (29825,29820), (29885,29880), (29945,29940), (30005,30000), (30065,30060), (30125,30120), (30185,30180), (30245,30240), (30305,30300), (30365,30360), (30425,30420), (30485,30480), (30545,30540), (30605,30600), (30665,30660), (30725,30720), (30785,30780), (30845,30840), (30905,30900), (30965,30960), (31025,31020), (31085,31080), (31145,31140), (31205,31200), (31265,31260), (31325,31320), (31385,31380), (31445,31440), (31505,31500), (31565,31560), (31625,31620), (31685,31680), (31745,31740), (31805,31800), (31865,31860), (31925,31920), (31985,31980), (32045,32040), (32105,32100), (32165,32160), (32225,32220), (32285,32280), (32345,32340), (32405,32400), (32465,32460), (32525,32520), (32585,32580), (32645,32640), (32705,32700), (32765,32760), (32825,32820), (32885,32880), (32945,32940), (33005,33000), (33065,33060), (33125