Problem Solving

At 50 mph a car uses 1 gallon for every 30 miles. If the car starts with a full tank of 12 gallons and drives for 5 hrs at 50mph, the amount of gas used would be what fraction of the tank?

oa: [spoiler]25/36[/spoiler]

Pretty straightforward question.

Car uses 1 gallon for every 30 miles. It went 5 hours at 50 m.p.h, so it went a total of 250 miles. It used a total of 250/30 or 25/3 gallons. If a full tank is 12 gallons, then the amount of gas used would be 25/3/(12) or 25/36 of the tank.

shibal wrote:At 50 mph a car uses 1 gallon for every 30 miles. If the car starts with a full tank of 12 gallons and drives for 5 hrs at 50mph, the amount of gas used would be what fraction of the tank?

oa: [spoiler]25/36[/spoiler]

Think of this question as a manipulation of units. Otherwise it is a direct rate/distance question.

5hrs*(50miles)/hr = 250 miles.

now you know the rate of gallons vs miles = 30 miles/gallon.

we want our end result to be in gallons.

so 250miles/(30m/g) = (25/3) gallons.

now they are asking you the fraction of 12 gallon tank used. both units are in gallons.

so (25/3)/12 = 25/36.