Of course, Ms. Forest could buy all packages of 9.

Or, Ms. Forest could buy all packages of 6.

But there are other quantities she could buy with some of each size package.

Notice that exchanging one package of 6 for one package of 9 increases the number of pencils by 3.

For part 3) notice that three packages of 6 can be traded for two packages of 9 without changing the quantity of pencils.