Write down a number you can square to give an answer bigger than 400 but smaller than 500

If you would like to know a number you can square and the answer is bigger than 400 but smaller than 500, you can calculate this using the following step:

20^2 = 400 < 21^2 = 441 < 22^2 = 484 < 500

There are two correct results: 21 and 22.