Thursday, 3 April 2014


Recently this question was posed to me:
How many times can you subtract 5 from 25?
 The supposed answer is once, because then you'd only have 20.

I see it differently. It's a calculation, and you can do the same calculation over and over again forever, if you wish. With an infinite amount of time and resources, you could do the problem forever indefinitely.

Let's consider something more realistic. If you were to do that calculation all day and night, without doing anything else, for the rest of your life, you would die in 3 days because you haven't had any fluid intake. If the calculation takes 2 seconds each time, you could do it 30 times in a minute, 1,800 times an hour, and 43,200 times in an entire day. So, in three days, you would do the calculation 129,600 times. Not too shabby.

Let's cut in time to eat and drink and sleep and survive. Let's assume that survival to a ripe old age means you have to spend 11 hours a day on it. That leaves 13 hours a day for calculating. In one day, this would be 23,400 times. And as life expectancy in the UK is about 80 years, if you did the calculation every day of your life, accounting for survival, you would do the calculation 683,280,000 times. That's quite a lot and it doesn't account for leap years, either, so it's slightly larger than that (or you decided to have a break every February 29th)

That's about how many times you could do the calculation.

Fun, isn't it?

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