The length of a text messaging conversation is normally distributed with a mean of 10 minutes and a standard deviation of 4 minutes. What is the probability that a conversation lasts longer than 20 minutes?

The standard of deviation is 4.

The mean is 10

20 minutes - 10 (the mean) = 10 minutes longer than the mean.

10 minutes longer / 4 minutes (deviation) = 2.5 standard deviations.

2.5 on the Z-table = 0.9938

The probability of being more than 20 minutes = 1 - 0.9938 = 0.0062 (0.62%)