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Hexadecimal (hex) to Decimal conversion requires briefly understanding number systems.

Decimal is the Base 10 system. Numbers count from 0 to 9 in the right-most column. That is ten permutations, hence the name, Base 10.
The first column on the right is the ones column.
Multiply it by the Base to get the next column -  the tens.
Multiply the tens column by the Base to get the hundreds column.
Multiply the hundreds column by the Base to get the thousands column.
So, 0123 is a decimal number composed of the following:
3 x 1 = 3
2 x 10 = 20
1 x 100 = 100
0 x 1000 = 0000
Add them all up and you get 0123.

In Hex, we use the Base 16 system. Numbers count from 0 to 9, then A to F in the right-most column. That is 16 permutations, hence the name, Base 16.
As with decimal, the first column on the right is the ones column.
Multiply it by the Base to get the next column - the sixteens.
Multiply the sixteens column by the Base to get the 256s column.
Multiply the 256s column by the Base to get the 4096s column.
So, 0x0123 as a hex number is composed of the following:
3 x 1 = 3
2 x 16 = 32
1 x 256 = 256
0 x 4096 = 0000
Add them all up and you get 0291.

Therefore, 0x0123 in hex is equal to 0291 in decimal.
AND THAT IS HEX TO DECIMAL CONVERSION.