# GRE® Counting Methods Practice

Counting all possible squares in a chess board

This GRE quant practice question is a permutation combination problem solving question. A classic counting methods question.

1. 64
2. 65
3. 4096
4. 1296
5. 204

#### Video Explanation

Scroll down for explanatory answer text

#### Use these hints to get the answer

1. There are more squares than the 64 1 x 1 squares.
2. Find out the number of 2 x 2 squares and so on up to 8 x 8 squares.

#### Number of squares in a chess board

There are more squares in a chess board than the 64 1 x 1 squares.
The squares start from 1 x 1 all the way up to 8 x 8.

Let us count them and find a way to add all of them.

1. 1 x 1 squares - 8 squares across the width and 8 squares along the length = 8 * 8 = 64
2. 2 x 2 squares - with the size of the square increasing by 1 square the number of squares across the width will be down to 7 and the ones along the length will also be down to 7. So, there are 7 * 7 = 49 (2 x 2) squares.
3. 3 x 3 squares - 6 squares across the width and 6 along the length = 6 * 6 = 36 (3 x 3) squares.
4. 4 x 4 squares - 5 squares across the width and 5 along the length = 5 * 5 = 25 (4 x 4) squares.
5. 5 x 5 squares - 4 squares across the width and 4 along the length = 4 * 4 = 16 5 x 5) squares.
6. 6 x 6 squares - 3 squares across the width and 3 along the length = 3 * 3 = 9 (6 x 6) squares.
7. 7 x 7 squares - 2 squares across the width and 2 along the length = 2 * 2 = 4 (7 x 7) squares.
8. 8 x 8 squares - 1 square across the width and 1 along the length = 1 * 1 = 1 (8 x 8) square.

Therefore, the total number of squares in a chess board = 64 + 49 + 36 + 25 + 16 + 9 + 4 + 1 = 204 squares.

If you figured that the number of squares is the summation of squares of natural numbers up to 8, you could have used the formula $$frac{n$n+1$(2n+1)}{6}), where n = 8.

#### Try this variant

How many rectangles are there in a chess board?

Hint: A rectangle comprises two horizontal lines and two vertical lines. Count the number of pairs of horizontal and number of pairs of vertical lines possible in a chess board to get the answer.