If 5 people buy 3 pens each and 3 people buy 7 pencils each, what is the ratio of the total number of pens to the total number of pencils?

15 : 21

Explanation

First, find the total number of pens: \(5 \times 3 = 15\)
Then, find the total number of pencils: \(3 \times 7 = 21\)
Finally, express it as a ratio 15 : 21