24 is 40% of what number ?

40% * x = 24 

We simply the 40% into either 2/5 (40/100) or 0.40. I will show both ways: 

(using 2/5) 

(2/5)x = 24 

We multiply each side by 5, and divide by 2 (or multiply by 5/2) 

(5/2)(2/5)x = 24(5/2) = (10/10)x = 120/2 

Simplified, we get x = 60. So 40% of 60 is 24. 

Here is the same problem again, this time written out with 0.40 instead of 2/5 

0.40(x) = 24 

(I like to multiply by 100 to get an even 40 instead of .40, so I am going to multiply each side by 100 to make things a little easier). 

40x = 2400 

Divide each side by 40 to isolate x 

x = 2400/40 

We simply to get x = 60, the same answer.  soo 24 is 40% of 60 :)