Calculator on order: basic maths and orders of magnitude

It's a sunny Saturday1, and two maths items have caught my eye.

Firstly, a bit of a cock-up on the sums front from Andover Tesco (via @reeveshall):

Eight grand for a kilo of blueberries? That can't be right. It's hard to figure out what they've done wrong here - a kilogram is only eight of their 125g servings, so the correct price is £8 per kilo. My first thought was "maybe they've confused grams and kilos?" - but the blueberries are about 0.8p per gram, not £8,000.2

The other story was even more amusing: the Spanish Navy has built a submarine that's 70 tonnes heavier than it ought to be, because someone missed a decimal point early in the design stages3. Oops. It's going to cost them somewhere in the region of ten million quid to put right. (They could, of course, use that money to buy buoyant blueberries.)

Back to Tesco, though; they responded, good-naturedly:

...and for a moment I was reassured. But then I thought, no: the problem wasn't with the calculator. It's with no-one thinking "8,000 punnets of these blueberries might come out to a bit more than a kilogram." It's with no-one saying "that number looks odd." And it's with everyone saying "It came out of the computer, so it must be right."

That's the kind of thinking that can cost you millions of pounds.

Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.

  1. at the time of writing. I don't know when you're reading this []
  2. On later thought: perhaps the weight was put in as 0.125g instead of 125g. That would explain it. []
  3. An error in their floating point arithmetic, as @realityminus3 says. []

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I teach in my home in Abbotsbury Road, Weymouth.

It's a 15-minute walk from Weymouth station, and it's on bus routes 3, 8 and X53. On-road parking is available nearby.

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