This showed up on digg, but turned out to be one of the most absurdly transparent “math magic trick” I’d ever seen. Imagine that your 7 digit phone number was written as abc-defg. This trick says to:

1. Enter abc into the calculator.
2. Multiply by 80. (Accumulator now contains 80*abc)
3. Add 1. (Accumulator now contains 80*abc+1)
4. Multiply by 250. (Accumulator now contains 20000*abc+250)
5. Add in defg. (Accumulator now contains 20000*abc+250+defg)
6. And add in defg again. (Accumlator contains 20000*abc+250+2*defg)
7. Subtract out the 250. (Accumulator contains 20000*abc+2*defg)
8. Divide by 2. (Wow! Accumulator contains 10000*abc+defg, your phone number! What a surprise!)

Weak. Really weak.

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