10 = 2/5 of what? How do you work the problem$

10 is 2/5 of 25.

Another way to write "10 = 2/5 of what?" is 10 = (2/5)*x where x is our variable.

To work this problem, we first multiply both sides by 5.  This gives
us 10*5 = 2*x since the 5's cancel on the right side.  Simplified, we
have 50 = 2*x.

We now divide both sides by 2.  This gives us 50/2 = x since the 2's
cancel on the right side.  Simplified, we have 25 = x.

10 = (2/5)x.  Solve for x.

Divide both sides by 2/5

10/(2/5) = 10(5/2) = x
25 = x

You can check it yourself.