Add the decimals, 3.8 and 8.7.

Hint:
We can add the decimals just as we add numbers, but with keeping the decimal point in the same place. The result must also have the same number of digits after its decimal point as the given decimals have.

Complete step-by-step solution:
We are given two decimals 3.8 and 8.7.
We have to find their addition.
3.8 is the sum of 3 and 0.8.
$3.8 = 3 + 0.8$
8.7 is the sum of 8 and 0.7
$
  8.7 = 8 + 0.7 \\
  3.8 + 8.7 = 3 + 0.8 + 8 + 0.7 \\
   = 3 + 8 + 0.8 + 0.7 \\
   = 11 + 1.5 \\
   = 12.5 \\
  \therefore 3.8 + 8.7 = 12.5 \\
$
The sum of 3.8 and 8.7 is 12.5.

Additional Information:
In mathematics algebra, a decimal can be defined as a number whose whole number part and fractional part is separated by a decimal point. The digits which are after the decimal point are fractions and the value of those digits is always less than 1. If the number has a decimal point, then the digits to the right of the decimal point indicates the number of tenths, first right digit is of one-tenth, second right digit is of one-hundredth and third right digit is of one-thousandth and so on. There are two types of decimals, Repeating and Non-repeating. In repeating decimals, the part after the decimal point repeats infinitely and in non-repeating decimals there is a termination.

Note:
If the no. of digits after decimal point varies with the number, then add extra zeros at the end of the decimal with less no. of digits to get the same number of digits for addition. Here the given numbers have the same no. of digits after their decimal points, so no need to add zeros.