Question

which deal on notebooks is the best 20 for $8 or 30 for $10

Answer 1

30 for 8 because for 2$ more you can have 10

Answer 2

So 20 for 8 would mean that each notebook Costs 40 cents
^ do 8 (money) divided by 20 (# of notebooks)
30 for 10 would mean that each notebook costs about 33 cents
^ do 10 (money) divided by 30 (# of notebooks)
So arguably, 30 for $10 is a better deal!
Hope that helped