Express $321_7-321_5$ in base 10.
To convert a number from base 7 or base 5 to base 10, I need to multiply each digit by the corresponding power of the base and add them up. For example, $321_7 = 3 \times 7^2 + 2 \times 7^1 + 1 \times 7^0 = 147 + 14 + 1 = 162$. Similarly, $321_5 = 3 \times 5^2 + 2 \times 5^1 + 1 \times 5^0 = 75 + 10 + 1 = 86$. Therefore, $321_7 - 321_5 = 162 - 86 = 76$ in base 10. 