You can treat multiplication as a means of finding the surface area of a rectangle. Breaking down that rectangle into other rectangles is perfectly valid.. provided you account for all of them.

For this case, you have a rectangle that is 61 by 31 units (feet, meters, inches, furlongs; whatever). You note that finding 60 x 30 is easier, so you divide it as such: 60 1 _____________________ | || | || | || 30 | || | || ---------------------- 1 |___________________||

You find the area of the big 60x30, which is 1,800 as you note. From there if you count a 1x61 and a 31x1 area you've counted the lower right 1x1 area twice. Alternatively, if you counted 1x60 and 30x1 then you wouldn't have counted that area at all. Your options are 61x31 = 60x30 + 61 + 31 - 1, or 61x31 = 60x30 + 60 + 30 + 1. Both of these give 1,891, the correct answer.