When you are turning a decimal number into a mixed number the numbers to the left of the decimal will become the whole number and the numbers to the right of the decimal will become the fraction.
for example: 5.815 + 6.021 = 11.836
11.836 is in decimal form
11.836 as a mixed number is
11 836/1000
The sum of the mixed numbers is 11 and 836 OUT OF 1000 compared to the sum of the decimals which is 11 and 836 1000's