Geri wants to put 6 stamps on some pages in her stamp book and 9 stamps on other pages. Explain how she could do this with 78 stamps.

If x = number of 6 stamp pages and y = number of 9 stamp pages, then we can use a formula.

6x + 9y = 78 stamps

Or ... You could divide the total number of stamps (78) by 9 which would give you 8 and 6 as a remainder. Therefore, 8 pages would have 9 stamps and 1 page would have 6 stamps,