Estimating first is one of the most important skills you can have when figuring out any problem. When you need to figure out the answer to additions of two or more digits, clever mathematicians ALWAYS estimate first in their head or even by writing their estimate down on paper.
Why should you always estimate first?
- It lets you check your answer in case you've made a mistake with the algorithm. This way, you don't need to rely on the teacher to check whether your answer is in the right ball park or not: you can check for yourself. Checking your answer against your estimate lets you know whether your answer is way off or likely to be correct.
- It gives you practice at working out answers in your head/mentally. This is often important when you go shopping to figure out the rough total or difference/change and in many other real-life situations.