Mr. McKay bought b boxes of crackers for $3 each and p pounds of cheese for $4 per pound. How much did he spend in all?

You can't solve it without another piece of information, so the most you can do is write an equation (3b + 4p), and once you know how many boxes he bought and the total (to figutre out how many pounds) or how many pounds and the total (to figure out how many boxes) or just the number of boxes and the number of pounds you can figure the problem out.