Final answer:
To estimate 635/39, round 635 to 600 and 39 to 40, then divide to get an estimate of 15.
Step-by-step explanation:
To estimate 635/39 by first rounding each number so that each has only one non-zero digit, we should round 635 to 600 and 39 to 40. Doing so, the estimation of the division would be:
600 / 40 = 15
Therefore, the estimate for 635 divided by 39, when each number is rounded to have only one non-zero digit, is 15.