$3\mathrm{.}3\mathrm{.}7$ When a two$-$way paging system transmits a message, the probability that the message will be received by the pager it sent to is p$.$ When the pager receives the message, it transmits an acknowledgment signal $($ACK$)$ to the paging system. If the paging system does not receive the ACK, it sends the message again.