The cost of diesel fuel varies directly with the number of gallons purchased. if 21 gallons of diesel fuel costs $73.50, how many gallons of diesel fuel can be purchased for $98?

The key here is the word "directly." We need to set up a ratio.

gallons / cost = gallons / cost

21 / 73.50 = gallons / 98

Cross multiply and simplify.

(21) (98) = gallons * 73.50

2058 = gallons * 73.50

2058 / 73.50 = gallons

28 = gallons

73.50:21=3.5 per gallon

98:3.5=28 gallons ... answer