1,2,3,4,5,6,7
问题描述：
1,2,3,4,5,6,7
how many pairs of different numbers can be chosen from the list above so that the sum of the two numbers is even?(note:the pair 1,3 is the same as the pair 3,1)
第二题
square tiles measuring 1/2 foot by 1/2 foot are sold in boxes containing 10 tiles each.what is the least number of the boxes of tiles needed to cover a rectangular floor that has dimensions 12 feet by 13 feet?
这道真心没看懂= =
答
1.Grp A (0dd numbers):1,3,5,7Grp B (Even numbers):2,4,6U can choose 2 both from Grp A or from Grp B,totally:C(2,4)+C(2,3)=92.12×13÷(1/2×1/2×10)=62.4≈63