Queue depth or no of packets in buffer=utilization/1-utilization.
Here utilization=Network load/ WAN Load.
Router having 5 multiple links and 4 are tiny flowing rate is=10000 packets/sec and another one is 50000packets/sec.
Consider entitre load on network it is $4*10^{4}+5*10^{4}$ ( 4 tiny flows and one huge flow).
Total load on network=9*$10^{4}$ packets .
but each packet is of length=10000bits.
So Total load on network=9*$10^{4}*10^{4}$=9*$10^{8}$.bits/sec
WAN load=1Gbps=$10^{9}$ bits/sec.
UTILIZATION=$\frac{9*10^{8}}{10^{9}}$
=0.9.
Queue depth=$\frac{0.9}{1-0.9}$=9.
Hence on an average no of packets in a buffer=9