A elevator has a capacity of twenty adults or twenty four childern. How many childern can ride with fifteen adults?

Let adults be x and children be y, then

20x = 24y

x = 6/5y

Let the number of children be a, then

15x + ay = 24y

15 (6/5y) + ay = 24y

18y + ay = 24y

18 + a = 24

a = 24 - 18 = 6

Therefore, 6 children can ride with 15 adults.