Two ways of finding out for sure. Easy to tell you whether 5/8 is greater or smaller than 5/12 but you need to be able to tell the teacher how you came to that conclusion.

Either convert them to decimals. 5/8 = 0.625 and 5/12 = 0.416666 recurring. Which is larger? 0.625, so 5/8 is greater than 5/12.

The other way is to give the fractions a common denominator (the bottom number on each fraction). Both 8 and 12 go nicely into 24, so make 24 the common denominator. Now convert the top number to correspond.

5/8 is the same as 15/24. Bottom number 8 goes into 24 three times, so multiply the top number 5 by three (15) 5/12 is the same as 10/24. Bottom number 12 goes into 24, two times. Now multiply the top number 5 by two (10)

So now you have 15/24 and 10/24. It's easy now to see that the first fraction is larger than the second fraction.