In the addition of two 2-digit numbers, each blank space, including those in the answer, is to be filled with one of the digits 0, 1, 2, 3, 4, 5, 6, each used exactly once. The unit's place digit of the sum is ______.

Seven digits are used in all, so the sum has three digits, and being the total of two 2-digit numbers it must start with 1 followed by 0. If the units digit of the sum is $$g$$, then the four digits of the two numbers add to $$20 - g$$, and comparing with the value $$100 + g$$ of the sum gives $$9 \times (\text{sum of the tens digits}) = 80 + 2g$$. Only $$g = 5$$ makes this a whole number, and $$42 + 63 = 105$$ shows it works, so the units digit of the sum is 5.