First, do 0.4 + 0.4. So, you can see what number you should get when you check your answer. For this, you should get 0.8
Then, do 0.8 + 1.6
After you add those 0.8 and 1.6, you should get your answer.
To check your answer, plug in your answer where B is and see if you get 0.8. I hope this helps! I did as much explaining as I could! If you need anymore help with this, just comment