Well a brilliant explanation has already been given, but I'm just sharing my interpretation that may make it a bit easier.
In token bucket algorithm, suppose if I want to send 1MB data, I need to have 1MB of tokens.
Here, we need to transmit 12MB of data, so we need to generate 12 MB of tokens.
The bucket capacity is 1MB and it has been stated that the bucket is full. So we already have 1 MB tokens available, now remaining only 11 MB of data. So, we require only 11 MB of tokens to be generated.
Now, token generation rate is 10 MBps:
10 MB ---> 1 sec
so 11 MB ---> ?
which is equal to 11/10 sec i.e. 1.1 sec
PS: Correct me if I'm wrong.