[ozaukeemom]

I must be an idiot because, no matter how many times I see that, I cannot understand it. I have no clue where these numbers are coming from or why? Inefficient???? I consider myself a bit of a numberhead, but this has me stumped.

[21twelve]

I actually do something like that in my head sometimes if I’m not too worried if I screw up. It is rounding up or down to an easier number and then adding in the numbers you added or subtracted.

BUT - hard to do if you don’t know your math facts (flash cards) - which they don’t teach any more.

[Slyfox]

I must be an idiot because, no matter how many times I see that, I cannot understand it.

Welcome to one of the many un-understandable essences of CommonCore.