Answer:
90
Explanation:
4/15 which is our fraction distance/time
our other fraction is 24/x (x is our variable to find our time)
Now we are doing the buultterfly method 
set it up 
now multiply 4*x= 4x 
15*24=360
now divide 4x ÷ 4 to get our variable alone 
do the same to the other side 
4 ÷ 360= 90 
so x= 90 which is our missing number