A 2 by 3 rectangle contains 8 squares. A 3 by 4 rectangle contains 20 squares. A 4 by 6 rectangle contains 50 squares. What size rectangle contains exactly 100 squares? Is there more than one? Can you find them all? Can you prove there are no more?

The rectangles that have exactly 100 squares are 1 by 100  4 by 11 and 5 by 8

Step-by-step explanation:

The given parameters are;

A 2 by 3 rectangle contains 8 squares

A 3 by 4 rectangle contains 20 squares

A 4 by 6 rectangle contains 50 squares  

We note that the number of squares varies proportionately to the increase in the dimension of the rectangle, that is we have;

2 by  3 rectangle contains 8 squares

3 by 4 rectangle contains 20 squares increase by 12 = 3 × 4

4 by 5 rectangle contains 40 squares increase by 20 = 4 × 5

The number of squares formed can therefore be arranged in a matrix as follows(please see attached matrix);

Column,     1,       2,     3,      4,       5,     6,        7,       8,         9,           10,         11

Row

1

2

3

4

5

6

7

8

9

10

11

It will be observed that after reaching a point where the row and column are equal, (x, x) the rate of increase in the number of squares become constant and equal to x × ((x - 1) × 0.5) + 1) = x × (x + 1)/2

From the matrix, the rectangles that have exactly 100 squares are 1 by 100  4 by 11 and 5 by 8 and there are no more as presented in the attached matrix, the number of squares formed keeps increasing for each column and row added.