math problem?

3581 × 863
≈ 4000 × 900
= 3600000

Says round each number so that it has only 1 nonzero digit:
Then 3851 -> 4000
And 863 -> 900
So 4,000 x 900 = 3,600,000
You can do it like this: 40 x 9 = 360 then add back the four zeroes removed.

 Estimate 3851 × 863 by first rounding each number 
 so that it has only 1 nonzero digit.
 3851 × 863 ≈ 4000 × 900 = 3,600,000

Sorry ... I can manually compute the true answer on a piece of paper faster than I can comprehend what this "only 1 non-zero digit" instruction expects you to do and yet EASILY estimate.   EDIT - I blame my bifocals for not reading the problem correctly (!)