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Is a deceasing linear model or an exponential decay model likely to be a better fit for the data about remembering? ?

The number of potato chips in a bag is normally distributed with a mean of 71 and a standard deviation of 2. approximately what percent of bags contain between 69 and 73 potato chips? approximately 68% approximately 71% approximately 95% approximately 99.7%

Can you me find the missing length. i attached an image.