郁闷又一题

tonylovecat

求大于700的三位数中偶数个数！特别要求every unit is non-zero,而且different

我算的解法是 2*C41*C71+C31*C71=77

新东方老师写出正版的解法（让我感觉是穷举）答案给的是75，请教Robert和各位高手,我的错误在哪里？[em27]

zhouxh

我觉得你的解法没错啊


你把原题给我看看。

nansresume

当百位7时，情况一:十位Odd(1,3,5,9)，个位（2，4，6，8）共 4*4=16
           情况二:十位Even（2,4,6,8),个位（2,4,6,8)共4*3=12
when hundreds units is 8, position 3, ten's unit is odd(1,3,5,7,9),then 个位(2,4,6),5*3=15
                          position 4, ten's unit is even(2,4,6),个位(2,4,6)3*2=6
when hundreds units is 9, the whole thing is just like the positions that when the hundreds units is 7.
So the answer is (4*4+4*3)*2+15+6=77
I am not sure that I am right