The average number of failures per week is: `mu=3/20=0.15`

"Not more than one failure" means we need to include the probabilities for "`0` failures" plus "`1` failure".

`P(x_0)+P(x_1)` `=(e^-0.15 0.15^0)/(0!)+(e^-0.15 0.15^1)/(1!)` `=0.98981`