What does estimating the sum mean?

Answer 1

It means to make an intelligent guess at an approximate answer rather than calculate the exact value.

For example, to estimate 1989 + 6019,

I would round 1989 to 2000 and 6019 to 6000 and so the sum simplifies to 2000 + 6000.

And that is 8000.

[The exact answer is 8008.]

Answer 2

It means round the number of each number you are adding i.e. 46 to 50 and 61 to 60. Then you just add them.

Answer 3

"Dose" is a measured portion of a medicine. I am not aware of any potion of medication that goes about estimating the sum!