Since we know that when the office sells 8 packs of envelopes for every 5 packs of paper, we can express the ratio of packs of paper to envelopes like this:
Then if we want to find how many packs of envelopes are sold with 15 packs of paper, we just have to multiply the number of packs of paper (15) by the ratio we expressed above, like this:
Then, 24 packs of envelopes are sold if the store sells 15 packs of paper