Suppose, household color TVs are replaced at an average age of \mu

Suppose, household color TVs are replaced at an average age of ${\displaystyle \mu =7.4years}$ after purchase, and the (95% of data) range was from 5.4 to 9.4 years. Thus, the range was ${\displaystyle 9.4-5.4=4.0}$ years. Let x be the age (in years) at which a color TV is replaced. Assume that x has a distribution that is approximately normal.
(a) The empirical rule indicates that for a symmetric and bell-shaped distribution, approximately 95% of the data lies within two standard deviations of the mean. Therefore, a 95% range of data values extending from ${\displaystyle \mu -2\sigma \to \mu +2\sigma }$ is often used for "commonly occurring" data values. Note that the interval from ${\displaystyle \mu -2\sigma \to \mu +2\sigma is4\sigma }$ in length. This leads to a "rule of thumb" for estimating the standard deviation from a 95% range of data values.
Estimating the standard deviation
For a symmetric, bell-shaped distribution,
${\displaystyle \text{standard deviation}\approx range4\approx highvalue4-lowvalue}$
where it is estimated that about 95% of the commonly occurring data values fall into this range.
Use this "rule of thumb" to approximate the standard deviation of x values, where x is the age (in years) at which a color TV is replaced. (Round your answer to one decimal place.)
(b) What is the probability that someone will keep a color TV more than 5 years before replacement? (Round your answer to four decimal places.)
(c) What is the probability that someone will keep a color TV fewer than 10 years before replacement? (Round your answer to four decimal places.)