Estimation is helpful when adding and subtracting decimals because it helps you know if your answer is close to the right answer or not. For example, say you added 12.4 + 2.73 and got 3.97. Rounding each number to the nearest whole number would give you 12 + 3, which equals 15. 3.97 isn’t anywhere close to that, so you know you must have made an error somewhere. You go back to your original work and notice that you forgot to align your decimal points. Now you can fix your problem and get the correct answer of 15.13.