I just received an email on a listserv I’m on with one of those “amazing” math tricks. The instructions say to “get a calculator” cause “you can’t do this in your head”. Grrrrr.
Here’s the “magic trick” -
Start off with your phone number, say 867-5309.
- Take the first three digits (867), multiply by 80.
- Add 1
- Multiply by 250
- Add the last four digits
- Add them again
- Subtract 250
- Divide by 2.
And be amazed at the result. Please.
The fact that this amazes anyone is a sad testament to our educational system with respect to mathematics. But in case you are math challenged, here, in four easy steps, is how it works (and you don’t need a calculator).
Step 1: combine the first three lines into one group - 250(80x + 1) where x is 867.
What you’ve done here is doubled the first three digits, moved them over four positions, and added 250. So our result is now 17340250.
Step 2: multiply the last four digits by 2 and add that to result from above.
You now have 17350868.
Step 3: Subtract 250 from above (remember we added 250 at Step 1, this just removes that
Result? 17350618
Step 4: Divide by 2 (remember that in Steps 1 and 2 we doubled the pieces of our phone number)
Presto, 8675309.
These things really make me sad. Innumeracy in action. For help, see this book:
Innumeracy: Mathematical Illiteracy and Its Consequences
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