Question

How to estimate

Answer 1

4 3/5 ≈ 5
1 1/5 ≈ 1

(4 3/5) +(1 1/5)
≈ 5 +1
≈ 6 . . . . . . . . . . . the estimated sum rounded to whole numbers

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Additional information:
As a refinement to your estimate, you can estimate the error. The first number is 2/5 less than 5. The second number is 1/5 more than 1, so the errors add up to
.. (2/5) +(-1/5) = 1/5

That is, your "estimate" of 6 is actually too high by 1/5. You can refine that estimated value by subtracting 1/5. 5 4/5 would be a closer "estimate."