GMAT

题目

GMAT
which of the following fractions has a decimal equivalent that is a terminating decimal?
a.10/189 b.15/196 c.16/225 d.25/144 e.39/128
时间来不及的!

答案

Notice:
1/2 = 0.5 (good)
1/3 = 0.3333.(bad)
1/4 = 0.25 (good)
1/5 = 0.2 (good)
1/6 = 0.16666...(bad)
1/7 = 0.142857142857...(bad)
1/8 = 0.125 (good)
1/9 = 0.11111...(bad)
1/10 = 0.1 (good)
1/11 = 0.0909...(bad)
etc.
189 --> 1 + 8 + 9 = 18 (multiple of 3)
196 --> 2 x 2 x 7 x 7 (multiple of 7)
225 --> 2 + 2 + 5 = 9 (multiple of 3)
144 --> 1 + 1 + 4 = 6 (multiple of 3)
128 --> 2 x 2 x 2 x 2 x 2 x 2 x 2
128 has only factors of 2 (or 5).
Answer:
E.39/128