Step-by-step explanation:
The same way that you can perform any (most) operation(s) on both sides of an equation, you can do the same to the numerator and denominator of a fraction. Thus, simply multiply the numerator (4) and the denominator (10) to get 40/100 from 4/10. Then, because 40/100 and 8/100 have the same denominator, simply add the numerators 40 and 8 to get 48/100.
Hope it helps :) and let me know if you want me to elaborate.