5 + 10.5 can be rewritten as 5.0 + 10.5. If you write this down to add them together, align the decimals and add like regular. Drag the decimal down for your final answer.
5.0 + 10.5 = 15.5
It is the same as doing 50 + 105 which would be 155 and adding the decimal (1 decimal place to the left) when we have the final answer.
