help needed please and thanks

A project's critical path is ABDE. The project activity information (in weeks) are as follows. Assume the project finish time follows a normal distribution. If the project deadline is set to be 28 weeks, then what is the z-value to determine the probability that the project will be finished before the given deadline? Numbers only. Keep 3-decimal if not exact, either round up or down is ok. 3.5 margin of error +/0.001