Hello!
First you have to make the fraction have the same denominator so they are related
You can multiply the 1 and the 3 in 1/3 by 5 to get 5/15
You can multiply the 2 and the 5 in 2/5 by 3 to get 6/15
So he painted 5/15 of his room with 6/15 cans of paint.
Next you have to find out how much more of the room that needs to be painted
You can multiply 5/15 by 3 to get 1 which is his whole room
Now you do 6/15 * 3 to see how much paint he needs
This gives us the answer of 18/15 which is equal to 1 and 1/6 cans of paint
Hope this helps!