Plz i am only in 4th grade and i don't understand

Okay so this is how you solve this.
 
4/5 + 4/7 = you can't do that so you gotta find the same bottom. 

 you find the same bottom by multiplying 5 and 7 which equals 35 

the you times the top by it's opposite. 

4*7 and 4*5 = 28, ,20 

you answer is 

28/30 + 20/30 now you just simply add 

28+ 20 = 48 

it'll be an improper faction 48/35

4/5 + 4/7 = 1 13/35

since 

4/5 + 4/7 = (4x7) + (5x4)/5x7

=48/35 = 1 13/35

(hope this helps.)