For a two-digit number, the first digit is twice the second. If you add the square of its first digit to this number, you get the square of some integer. Find the original two-digit number.

#
The first digit is twice the second only in the following two-digit numbers: 21, 42, 63, and 84. By checking, we find that only the number 21 satisfies the condition of the problem.

## Answer

21.00