A source needs to transmit data packets to a receiver over a radio link. The receiver uses error detection to identify packets that have been corrupted by radio noise. When a packet is received error-free, the receiver sends an acknowledgment (ACK) back to the source. When the receiver gets a packet with errors, a negative acknowledgment (NAK) message is sent back to the source. The packet is retransmitted each time the source receives a NAK. We assume that each packet transmission is independently corrupted by errors with probability q. Find the PMF of X, the number of times that a packet is transmitted by the source.