40,000 is a reasonable estimate for 38,442 and 500 for 465. What I've done is to round up 38,442 to the nearest 10,000 and round up 465 to the nearest 100.
To divide 40,000 by 500, which both have two zeroes at the end, is to take them off both numbers, so we actually divide in our heads 400 by 5, which is 80.
So 80 is the best estimate for the result. Let's see how good it was: 38442/465=82.7 approximately, so the estimate is quite close.