Ans: $100$ $b$
Token bucket means, there will be a token rate (r), token capacity (b), and packet size (n).
If bucket contains atleast n tokens, then the tokens are removed and packet is sent, else nothing is done.
In this problem, the capacity is not provided, but the output rate mandates a the packet size.
This is because, if you have data < 8500kb then even if you flush all that in next sec, you can’t match 8500kb
Since $250 \times 24$ $+$ $120 \times 20$ $=8400$ $kb$ $\lt$ $8500$ $kb$
The bucket is short of $100$ $b$ to produce required burst.
So, the capacity is atleast $100$ $b$ more.