Three of four numbers have a sum of 22. if the average of the four numbers is 8, what is the fourth number? don't just answer, tell me how to answer. i think i just need setting up the correct equation

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.

The length of a rectangle plus its width is 25 cm. the area is 156 square cm. what are the length and width of the rectangle? separate the answers with a comma.

Three times the sum of a number and seven is negative thirty-six. what is the number?