Question

find the missing digit in 2220 dash so that the number becomes a perfect square

Answer 1

Square numbers only end in 1, 4, 5, 6, 9, 0. And the largest digit is 9. Thus, the largest possible perfect square is 22209. Let's Take the square root of 22209. It's a little over 149. Therefore, the perfect square must be 149^2 which is 22201, so the missing digit is 1.