standard deviation $7\mathrm{.}5\%.$

a$.$ For what proportion of these corporations was the rate of return higher than $20\%?$

b$.$ For what proportion of these corporations was the rate of return negative?

c$.$ For what proportion of these corporations was the rate of return between $10\%$ and $20\%?$

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a$.$ The proportion of corporations whose rate of return was higher than $20\%$ is $($Round to four decimal places as needed.$)$