Double 14.5 is 29, which is close to 28.5. Roman's car gets slightly less than 2 time as many miles per gallon as Lei's car.
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If you like, you can take the estimate to the next level. The estimate of "2 times" is high by a factor of 29/28.5 or by 1 unit in 58. This is between 1.5% and 2% high, so a closer estimate is between 1.96 and 1.97 times as many.
