Performance Perspectives Blog

Unless you know the rules, how can you get it right?

by | Feb 17, 2013

A Facebook friend of mine responded to the following mathematical puzzle:

To learn that you have to be a genius to solve it can cause some anxiety. But in reality, it’s quite simple, though most of those who tried got it wrong. Most people, I fear, are either uncomfortable with algebraic expressions or forget what they were taught. It’s handy to recall the mnemonic, Please (parenthesis) My (multiplication) Dear (division) Aunt (addition) Sally (subtraction).

You don’t do a mathematical expression like this from left to right; rather, you evaluate what is shown to decide the order.

0 + 50×1 – 60 – 60×0 + 10

Multiplication, first, so 50×1 =50 and 60×0 = 0
And so we then have:

0 + 50 – 60 – 0 + 10 = 0

Don’t believe it? Go to Excel and key in =0+50*1-60-60*0+10 and see what you get.

Seeing so many get this wrong suggests that most people simply go from left to right; but that’s not the way math works.

This same page offered a similar puzzle:

While their claim that 92% fail seemed to be high, given the number who responded and got it wrong suggests that it’s probably about right. Again, we evaluate the expression before trying to solve; don’t just go from left to right.

There are fundamental rules in mathematics, that if not obeyed, yield incorrect results. Likewise, we have fundamental rules in performance measurement. Disobey and you will err.

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