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casting out 9's....

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My kids have had to use it in factoring. It's a quick way to see if 9 is a factor of a big number, along with the other tricks for other numbers, and I don't allow calculators until algebra 1, and then only for super messy numbers. There are lots of times a calculator is slower than mental math, in real world problem solving. Just my two cents, I know I am pretty old fashioned. But my father can solve most complicated math problems in his head faster than I can get out a calculator and plug in the numbers, and it keeps one's brain exercised as well. My dh is faster than my calculator too, though not quite as fast as my father. I didn't get quite that good an education in mental math, but I want that for my kids. He was in one of those tiny country schools a long time ago where they had to do compound interest problems in their heads and such. Amazing.

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