b, 37 x 39 + 78 x 14 + 13 x 85 + 52 x 55
= 37 x 39 + 39 x 2 x 14 + 13 x 85 + 13 x 4 x 55
= 39 x (37 + 2 x 14) + 13 x (85 + 4 x 55)
= 39 x (37 + 28) + 13 x (85 + 220)
= 39 x 65 + 13 x 305
= 13 x 3 x 65 + 13 x 305
= 13 x (3 x 65 + 305)
= 13 x (195 + 305)
= 13 x 500
= 6500