Question

The length of a text message is conversation is normally distributed with a mean of 2 min and a standard deviation of 0.5 min.

Answer 1

$\mathbb P(X>3)=\mathbb P\left(\dfrac{X-2}{0.5}>\dfrac{3-2}{0.5}\right)=\mathbb P(Z>2)$

About 95% of a normal distribution lies within two standard deviations of the mean, so 5% lies without, and since the distribution is symmetric, 2.5% lies to either side. This means

$\mathbb P(X>3)=\mathbb P(Z>2)\approx0.025$