Consider this:

When the number of homework in grade six is 3, 1/3 is 1, 1 is 1/2 of the number of homework in grade five, and the number of homework in grade five is 1*2=2.

So the homework in grade five is two-thirds of that in grade six.

One * * * is 2+3=5 copies, and each copy is 120÷5=24 copies.

Grade 5: 24*2=48 books.

Grade 6: 24*3=72 books.

This may be easier to understand, I hope it will help you!