How to Check a Calculator's Accuracy. Many people think that unless a calculator shows some gross abnormality, then it is performing correctly. Unfortunately, this is not true. A calculator can appear to be functioning normally and actually be erring its functions without its operator's knowledge. Yes, electronics can make mistakes! But, engineers have a simple test that will check for such errors, and they usually begin a work day by performing it. This test does not require the manipulation of complicated formulas. It uses a "magic number."
The magic number is "370." Enter this magic number, 370, into your computer. Multiply it by 3. The computer total should show 1110.
Enter 370 into your computer once again. Multiply it by 6. The computer total should show 2220.
Enter 370 into your computer a third time. Multiply it by 9. The computer total should show 3330.
Enter 370 into your computer a fourth time. Multiply it by 12. The computer total should show 4440.
Realize that multiplying 370 by a number divisible by 3 will produce a total that has the multiple of three as its first 3 digits. For example, 27 is 9 threes If you multiply 370 by 9, the result is 9990.