Here is the number of 6 cm squares.

Suppose there are n (6) squares, then the number of (5) squares is N+99.

36-25 is an area whose (6) square is larger than a (5) square.

So 99 (5) squares are areas where N (6) is greater than N (5).

Divide the total extra area by each extra area to get the number (6).

225×(6×6)=8 100

You can't express it clearly in words. If you still don't understand, I suggest you use the equation:

Let the square of (6) be x.

36X=25(X+99)

.. this may be easier to understand.