[goodnesswins]

That's the way to do math in your head easily.....and I'm NEVER surprised that they do NOT teach this stuff in schools. It seems so many of the teachers use NO imagination when teaching, except, of course, when helping students explore their sexuality.

[Diddley]

.....and I'm NEVER surprised that they do NOT teach this stuff in schools. . . . no imagination . . .

Many don't know the subject well (they can't use it with ease) and thus they don't really know how to teach it, IMO.

[RJayneJ]

My husband has always done this and until High School he was in a one room schoolhouse. He is a home builder and for a quick idea of the cost of an option he just needs a close cost where precise isn't necessary. He can have it solved while I'm still looking for a pen and pad. };^D)
Actually, I do the same thing when quilting and what not.