Input burst = 50KB/sec for 10 sec and 10KB/sec for 50sec.
Output rate = 5KB/sec
Total time = 60sec ,Total Length = 500KB + 500KB = 1000KB.
Now output rate = 5KB ------ 1 sec
? ------ 60sec
In 60sec we can transmit 300KB.
We should have a Bucket of size 1000KB - 300KB = 700KB