TOM will from intersection X to intersection Y along a route that is confined to the square grid of four streets and three avennues shown in the map above. How many routes from X or Y can Tom take that have the minimum possible length.
Ok, there are five step to the destiny, three ||| and to__ __. so 5 elements to 排列 it is P(55), but ||| are same, then divied by P(33), two __ __ are same ,then divided by P(22),
finally the answer is P(55)/P(22)/P(33)=10.
