575
Subtraction means distance on the number line. From 64 to 100 is 36, from 100 to 600 is 500, and from 600 to 639 is 39. Therefore, 639 - 64 = 500 + 36 + 39 = 500 + 36 + 40 - 1 = 500 + 75 = 575. --- Alternately, treat the one's digits separately from the others: 630 - 60 = 570, and 9 - 4 = 5, so 639 - 64 = 575.