​ What is the population of the bacteria after 40 hours?

We have this

P  =  40 *  2^(t/15)      where  P   is the population after   t  hours

We want to find out the population after 40  hours  ....... so....

P  = 40 * 2^ (40/15)  

P = 40 * 2^(8/3)    ≈   254