单选题
If gasoline costs $2.79 per gallon, then approximately how much will it cost to drive 100 miles in a truck that gets 18 miles per gallon?
【正确答案】
A
【答案解析】The three numbers in this problem, written as fractions with units, are 
, 100 miles, and 
. If you multiply the first two by the reciprocal of the third, all of the units except for the dollars will cancel: 
, 100 miles, and . If you multiply the first two by the reciprocal of the third, all of the units except for the dollars will cancel: