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%)
