## 12 3/10 = 12.3##
Look at what you already know:
The whole number is 12 that will be written to the left of the decimal point.
The fraction has its denominator as a power of 10 which means it can be written as a decimal with 1 decimal place. ##3/10 = 0.3##
##:. 12 3/10 = 12.3##