The cost of pencils, C, varies directly as the number of boxes, B, purchased. If 4 boxes cost $10.68, then how much will 9 boxes of pencils cost?

When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.