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When I was 12 I got a new stepfather. He was a good guy, an engineer, and great with math.

One thing I started to notice about him as that he was especially good at estimating numbers. We’d be having some random conversation like what the monthly payments would be on $4902 spread over 12 months and he’d say “four hundred-ish”. Then I’d do the actual math and tell him he was wrong, the correct number was $408.50! But by then he’d moved on to some other pressing topic to discuss.

It was a real eye-opener for me, because if I wrote “four hundred-ish” as the answer on a 7th grade math test, my teacher would have marked the answer wrong (and probably given me a lecture about my attitude). And yet here was a very successful engineer using approximate numbers to quickly get to a decision.

I was thinking about this the other day, as I was working with one of the social entrepreneurs in the . My co-mentor asked our entrepreneur “How many units can you make a year, ish”. And our entrepreneur responded “Probably 80-ish”. And this went back and forth for various other key metrics associated with the venture (with an “ish” added to each number) and we quickly identified the scaling model that made the most sense for this startup. All those number will need to be double-checked, of course, but we were able to quickly get to the decisions we needed to make using “ish math” in our heads.

The reality is that for most decisions in life (and certainly most decisions in business) an approximate answer is all you need. And being able to rapidly get to the approximate answer in your head allows you to make a good decision and move on. That’s a really important life skill.

So that’s my advice for kids today. You are growing up in a world where calculators can give you an accurate number down to 12 decimal places — but that doesn’t necessarily get you to good decisions, and can sometimes even lead to bad decisions (see: ). Estimating the relationships between numbers, and understanding the outcome you are looking for, is what drives good decisions. It’s the power of “ish”.

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I’m a Silicon Valley guy. I teach entrepreneurship at Stanford, coach startup CEO’s at Miller Center, and run the 4thly Startup Accelerator. Also, I love tacos.

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