University of Toronto mathematician Ed Barbeau tells of a Montana student who was asked to find the cost of fencing the perimeter of a rectangular field 132 feet long and 99 feet wide, given that there are 16.5 feet in a rod and that one rod of fencing costs $12.75. The student first worked out five products:

165 × 4 = 660
165 × 7 = 1155
165 × 6 = 990
165 × 9 = 1485
165 × 8 = 1320

He crossed out all but the third and fifth of these and continued:

1275 × 6 = 7650
1275 × 8 = 10200
10200 + 7650 = 17850
17850 + 17850 = 35700

He gave 35700 as the answer. “Indeed the required cost is $357.”

(Ed Barbeau, “Fallacies, Flaws, and Flimflam,” College Mathematics Journal 37:4 [September 2006], 290-292.)