451
Subtraction means distance on the number line. From 66 to 100 is 34, from 100 to 500 is 400, and from 500 to 517 is 17. Therefore, 517 - 66 = 34 + 400 + 17 = 451. --- Alternately, treat the one's digits separately from the others: 510 - 60 = 450, and 7 - 6 = 1, so 517 - 66 = 451.