I only need help with B the answer to A was 680 us. The answer to B 5.882 MBPS but I need help with the steps thank you

The propagation delay between any two hosts (A, B or a switch) is 20S. The frame to transmit from node A to node B consists of 4,000 bits. Each switch can start retransmission of a frame 10S after receiving the last bit of a frame. a) What is the time necessary to transmit the data as a single frame from A to B (time from the first bit of the frame transmitted by node A until last bit of the frame is received at node B )? b) What is the effective data throughput rate for this one frame from A to B (number of bits sent divided by time to send the bits) in bits per second(bps) for the network as analyzed in part a? (answer is less than 1Gbps )