GRE PP2数学一题提问~

题目

GRE PP2数学一题提问~
题目是这样的： A rectangular game board is composed of identical squares arranged in a rectangular array of r row and r+1 columns. The r rows are numbered from 1 through r, and the r+1 comlumns are numbered from 1 through r+1. If r>10, which of the following represents the number of squares on the board that are neither in the 4th nor the 7th column?
原题给的答案选项是r2-r请问这题应该怎么想呢?没太看懂题目意思.请大家帮忙解答~多谢!

答案

本题选a!
就是r*(r+1)格中去掉第4行第7列的格子后还有几格,
所以
共有:r*(r+1)-(r+r+1)+1
=r^2+r-2r-1+1=r^2-r.