4/10 can be simplified to 2/5. 1/5 is equal to 20% (you probably want to memorize that) so all you have to do is multiply 20% by 2 (because of the numerator) and get 40%
So therefore 4/10 = 2/5 or 40%
P.S if you have anything over ten that needs to be a percent just multiply 10% by the numerator.
2/5 as a simplified fraction and 40 percent
C. identifying the problem