[求助]OG P178第195题

我就是我

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Pat will walk from intersection X to intersection Y along a route that is confined to the square grid of four streets and three avenues shown in the map above. How many routes from X to Y can Pat take that have the minimum possible length?

A. 6      B. 8       C. 10       D. 14          E. 16 

我不明白10是怎么算出来的？

我就是我

不好意思,我忘画了一条线,就是Avenue B

村民乙

反正走过去要走三次横的，两次竖的，五次总共，无论先后顺序。五个位置，里面排列三个横，两个竖，多少种排法？相当于五个位置排列两个竖，5*4/2=10