Let’s assume the breadth of the rectangle is x.

According to the given information, the length of the rectangle is 20% more than its breadth, which means the length is x + 0.2x = 1.2x.

The area of the rectangle is given by length multiplied by breadth, so it is (1.2x)(x) = 1.2x^2.

The side of the square is equal to the breadth of the rectangle, so it is x.

The area of the square is given by side squared, so it is x^2.

The ratio of the area of the rectangle to the area of the square is (1.2x^2)/(x^2) = 1.2.

Therefore, the ratio of the area of the rectangle to the area of the square is 6:5.

Hence, the answer is 6:5.