Question

How many total squares are there in a 10×10 grid?
(Clue: The answer is not 100. It is greater than 200.)

Answer 1

Answer:

There is obviously only 1 10x10 square. If we start with a 9x9 square in the top left corner, we can move it down 1, across 1 and back up 1, so there are 4 possible 9x9 squares.

For 8x8 we can move down 1, then 2 and we can move across 1 or 2 so that's 9 possible 8x8 squares.

For 7x7 we can go down 3 and across 3, so that's 4x4=16 possible squares.

The pattern is now clear the total number if squares is 1+4+9+16+…+100.

There's a formula for this, which I had to look up, but any the sum of the first n squared is 1/6n(n+1)(2n+1)1/6n(n+1)(2n+1), so the total number of squares is 10x11x21/6=5x11x7=385.