Someone please help me on question b

Which value of c would make he following expression completely factored? 8x+cy

Trent wants to buy 2 packs of trading cards for 3 dollars each. the trading card packs that trent normally buys tend to come in packs of 6, 10, 12, or 15 cards. after selecting 2 packs, trent found that the first pack of cards cost 25 cents per card, and the second pack cost 30 cents per card. trent uses this information to write the equations below in order to compare c, the number of cards in each pack.

Compare the cost of the bike trips from the two companies shown.for both the cost in a linear function of the number of days which statements is true