Question

How to estimate to check that the answer is reasonable

Answer 1

Basically estimating is rounding, im going to futher explain this with an example.
For example:
If I wanted to round a number by it's nearest tenth such as 678
(*example)
there are 70 tenths
now when you are estimating when you are rounding to the nearest number if the number has a higher value then 5 you increase the number if it is lower than 5 you STAY the same.

now
678,
since the number next to it is 8, 7 increases and everything else turns into zero. so 678 rounded to the nearest 10th
is 680!

Hope this example helped!