PP1-15-PS

[pimi]

15. 2160-!-item-!-187;#058&002594
At a speed of 50 miles per hour, a certain car uses 1 gallon of gasoline every 30 miles. If the car starts with a full 12 gallon tank of gasoline and travels for 5 hours at 50 miles per hour, the amount of gasoline used would be what fraction of a full tank?

(A) 3/25
(B) 11/36
(C) 7/12
(D) 2/3
(E) 25/36

Ans:E

我是這樣算的總距離5hr x 速率50mile=250
1 gallon of gasoline every 30 miles, 所以250/30可以算出花了多少gallon的油, 題目要算用的油佔a full tank(12 gallon)多少？
250/30=8.33.....
8.333..../12=答案

算法我都知道, 只是這題除不盡, 如果直接四捨五入用8/12去算答案, 會選到D
當然認真的把8.333..../12去運算, 會發現答案的確等於E 25/36
但是考試這樣算也太耗時間了吧, 通常為了求運算速度快, 小數點都會四捨五入以求最接近的答案, 哪知道這題那麼討厭, 偏偏D跟E答案是那麼的接近, 哀哀哀～
想請問大大, 有沒有更有效率的算法呢？？？？

[layeon]

油耗 = 30 mile/gal
travel 5 hrs * 50 miles/hr = 250 miles
總共耗油 = 250/30 = 25/3 gal
問題問: 總共油耗佔滿油箱的幾分之幾 = 25/3 除以滿油箱 12 = 25/36 - 答案E

[pimi]

我真是白癡 @ @