To add these fractions, we need to find a common denominator. The least common multiple of 24 and 21 is 168.

18/24 = (18/24) * (7/7) = 126/168
15/21 = (15/21) * (8/8) = 120/168

Now we can add the fractions:

126/168 + 120/168 = (126 + 120)/168 = 246/168

The fraction 246/168 can be simplified by dividing both the numerator and denominator by their greatest common divisor, which is 6:

246/168 = (246/6) / (168/6) = 41/28

So, 18/24 + 15/21 = 41/28.