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.