The answer will depend on what you wish to do with the estimated number. If, for example you wanted to multiply by 15.3, then I would suggest 140000. Then 137638*15.3 = approx 140000*15 = 2100000. If, on the other hand, you wanted to subtract 5600 then I would suggest 137600. Then 137638 - 5600 = approx 137600 - 5600 = 13200.
The answer depends on what you wish to do with the estimated number. If dividing by 15 I suggest 135000 because 135000/15 = 9000. If multiplying by 142, then I would suggest 140000, since 137638*142 = approx 140000*140 = 19600000.