50.249 + 21.3 can be written as (50 + 20) + (0 + 1) + (0.2 + 0.3) + 0.049, which makes calculating the sum easier. 50 + 20 = 70, 0 + 1 = 1, 0.2 + 0.3 = 0.5, and you still have a 0.049. Add the numbers all together and you'll get that the sum of 50.249 and 21.3 is 70 + 1 + 0.5 + 0.049 or 71.549.
