The cost of gasoline varies directly with the number of gallons bought. If 5 gallons of gasoline cost $18.00, find the cost of 20 gallons.

You would divide 18 by 5 to find the unit rate which is cost per gallon. 18/5=3.60$ So gas is 3.60$ per gallon (unit rate). So to find the cost of 20 gallons you would multiply 3.60 by 20 which equals 72$. So 20 gallons of gas costs $72.