Firstly, put both decimal numbers into simplest fraction form:
0.5 = 1/2 0.10 = 1/10
To add these, you need to have the denominator (the bottom number) the same on both fractions. You use the smallest number you can get, which is called the lowest common denominator, or LCD.
The LCD in this case is 10, as the second fraction cannot be written with any smaller denominator than 10. So you need to make the fraction 1/2 have a denominator of 10.
10/2 is 5, so you multiply both the numerator (top number) and the denominator by 5: 1 x 5 / 2 x 5 = 5/10
Now you can add the two fractions together: 1/10 + 5/10 = 6/10
If you want to, you can then simplify this answer: 6/10 (divide both numerator and denominator by 2) = 3/5
