Starting with one bacteria, we will have 8 bacteria in 1 hour. After that we will have 15 - 1 = 14 hours more. In 1 hour we have 3 times 20 minutes ( bacteria can multiply every 20 minutes ). 14 x 3 = 42. It means that there are 42 periods of 20 minutes more.
The number of bacteria after 15 hours:
8 x 2^(42) = 2^(45) = 35,184,372,088,832.