31A (base sixteen) = 3 × sixteen2 + 1 × sixteen1 + A × sixteen0
To convert a hexadecimal representation of an integer to base 10, write the integer as a sum of hex digits times the power of 16 that corresponds to each digit's position. Then, write the digits and the powers of 16 in base ten, and do the arithmetic.
31A (base sixteen) = 3 × sixteen2 + 1 × sixteen1 + A × sixteen0 = 3 × 162 + 1 × 161 + 10 × 160 = 3 × 256 + 1 × 16 + 10 × 1 = 79410
You don't have to remember powers of 16 to do this conversion.
All you need is powers of 2.
For example,
As another example: 163 = 24 ×24 ×24 = 212 = 22 ×210 = 4K
More practice: What integer is represented by 1B2 (base sixteen) ?