Pleade HELP IM DO STUCK ON THIS SOMEONE ANSWER ITS ALMOST DUE

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.

He that is measured at 220° is a reflex angle. the opposite angle is obtuse. find the measure of obtuse .

How is working backward similar to solving an equation?