10c - f = -13 cd solve for c

Having a hard time understanding this question (see attachment).

Adoughnut shop charges $0.50 for each doughnut and $1 for a carryout box. shirley has $10 to spend. which inequality can be used to find d, the number of doughnuts she could have bought if she also wants them in one carryout box?

Bob paid $3 less than tim at a local pizza parlor. together they spent $7.80. how much did bob pay for pizza?