For 5s, data are sent with maximum bandwidth after that inflow rate = outflow rate = x Mbps.
Here C is the capacity of the bucket and the inflow rate is xMbps.
Maximum data can be sent in 5s : C + 5x
Maximum average rate: $\frac{C +5x}{5}$
$\frac{C +5x}{5}$ = 20
=> C + 5x = 20 X 5
=> C + 5x = 100
So data sent till now is 100 Mb for 5s.
Also, given in the question “ atmost 150 Mb can be sent over 10 sec”. Left time is 5s.So, for sending remaining 50Mb, we have time 5s.
Now outflow rate = inflow rate. So, outflow rate is x Mbps.
Therefore, $\frac{50}{x}$ = 5
=> x = 10
Therefore refresh rate is 10Mbps.