Computer A has 30 MB to send and it will send this data at the burst rate of 6Mbps
6Mb = 1 sec
1Mb = 1/6 sec
30 x 8 Mb = (30x8)/6 sec = 40 second
and the routers are using a leaky bucket so they will transmit at the rate of 4Mbps
in 40 sec routers will transmit = 40x4 = 160Mb
Remaining data = 240 – 160 = 80 Mb
this 80 Mb is stored inside the buffer because the router is unable to send this in 40 seconds.