Adding Decimals

Today I did a DLO showing how to find out how to solve a Decimals so I add .4 to the 128.6 and take away .4 from 120.9. Under the top one I add 100 + 100 = 200 then I add 2 + 2 = 4 then I add 9 + 0 = 9 the answer was 249.5.