Question $28$

$2\mathrm{.}5$ pts

A study of a company's practice regarding the payment of invoices revealed that an invoice was paid an average of $20$ days after it was received. The standard deviation equaled five days. Assuming that the distribution is normal, what percent of the invoices were paid within $15$ days of receipt?

$15\mathrm{.}87\%$

$34\mathrm{.}13\%$

$86\mathrm{.}74\%$

$37\mathrm{.}91\%$