Estimating helps check the reasonableness of your answers because it gives you an idea of the range your answer is in. For example, let's take the expression 121 + 54, which is 175. If you estimate, it will be 120 + 50, which gives you 170. By doing this, you know that your answer is reasonable because 175 is fairly close to 170. If you are off with your calculation, let's say you get the answer 1750. By estimating, you would know that your answer is u reasonable, as 1750 is not close to 170 at all
