To estimate 586 - 321, you can round the numbers up or down to the nearest five.
586 can either become 570, rounding up, or 585, rounding down, which is closer. 321 becomes 320. It's not close enough to 325, nor to 315, for either of those to be good choices. 570 - 320 = 250 and 585 - 320 = 265. The actual solution to the original problem is 265, so, the second estimate, because they both go down by 1, is the closest.