Simple way :
Token rate into bucket = 2 Mbps
Output rate of bucket = 10 Mbps
Effective rate = 10-2 = 8 Mbps
Bucket capacity is full, 16 Mb
So, at effective rate bucket will drain in 16/8 = 2s
In these 2s, output capacity of bucket will be 10Mbps, after bucket gets empty, it's capacity will reduce to effective rate of 8Mbps.