feifei 1-17

[zeta1203]

17、m is two digit number and 2m is a three digit number, what is the unit digit of m?
（1）the unit number of 2m is 4;
（2）the unit number of m is the same as the tens number of 2m；

【答案】E
【思 路】（1）表示the unit number of m可以是2或7...如何得知?
（2）沒有去除任何可能，舉例m可以是62或87。..?

謝謝!拜託

[zeta1203]

[amazingslim]

17、m is two digit number and 2m is a three digit number, what is the unit digit of m?
（1）the unit number of 2m is 4;
（2）the unit number of m is the same as the tens number of 2m；

【答案】E
【思 路】（1）表示the unit number of m可以是2或7...如何得知?
（2）沒有去除任何可能，舉例m可以是62或87。..?

謝謝!拜託

m乘以2的個位數是4
m的個位數
有兩個可能:
(1)4/2=2

(2)個位數還是4，只是進位了
14/2=7

因為有62或87兩個數符合，所以答案為E