A car goes 15 miles on a gallon of gas

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Question from Raven:

A car goes 15 miles on a gallon of gas when it goes 50mph. When the car is driven 60mph it only goes 80% as far.
How far will it travel on a gallon of gas at 60mph?
I solved this two ways:
15 * .80 = 12 It goes only 12 miles or x/60 = 15/50 which is x = 12

This answer in the book is 4/5 * 15 = x. Then x = 12
How did they come up with 4/5?

Thanks and regards,
Raven

Raven,

You wrote 80% in decimal form as 0.80 but as a common fraction 80% is 80/100 = 8/10 = 4/5.

Penny

Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.