A project has four activities $($A$,$ B$,$ C$,$ and D$)$ that must be performed sequentially. The probability distributions for the time required to complete each of the activities are as follows.

ActivityActivity Time $($weeks$)$ProbabilityA$50\mathrm{.}3060\mathrm{.}3570\mathrm{.}2080\mathrm{.}15$B$30\mathrm{.}2550\mathrm{.}5070\mathrm{.}25$C$100\mathrm{.}10120\mathrm{.}25140\mathrm{.}45160\mathrm{.}15180\mathrm{.}05$D$80\mathrm{.}65100\mathrm{.}35$