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A made-up study measures the reaction times of 1000 people…

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Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

A made-up study measures the reaction times of $1000$ people. The times follow a normal curve with mean $250\ \text{ms}$ and standard deviation $30\ \text{ms}$ . Use the rule, split into its pieces of $34\%$ and $13.5\%$ , to find about how many people have a reaction time between $220\ \text{ms}$ and $310\ \text{ms}$ .

Need a reminder? The key ideaShow

A normal curve is fixed by $\mu$ and $\sigma$ . About $68\%$ , $95\%$ and $99.7\%$ of values lie within one, two and three standard deviations of the mean (exactly $0.6827$ , $0.9545$ and $0.9973$ ).

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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