Regina has two electronic files. one has a size of 3.15 mb and the other has a size 4.89 mb. what is the best estimate of the total size of the two electronic files?

So,

If you are rounding to the nearest tenth of a MB, then it would look like this:
3.2 + 4.9 = 8.1 MB
Because in 3.15, 5 is greater than or equal to 5, and in 4.89, 9 is greater than or equal to 5.

If you are rounding to the nearest MB, then it would look like this:
3 + 5 = 8 MB
Because in 3.15, 1 is less than 5, and in 4.89, 8 is greater than or equal to 5.

In my opinion, I would mentally add them to get 8.14 MB, because if you do math well, you can mentally add these things like second nature.  This is really helpful when you go shopping, which I'll bet you will be doing profusely.