Hey, what you described is an example of exponential growth. Let me show you a pattern in what you're describing:
Gen 1: (2*1.5)=3
Gen 2: (3*1.5)= (2*1.5)*1.5 = 2 * (1.5)2 = 4.5
Gen 3: (4.5*1.5) = 2 * (1.5)2 * 1.5 = 2 * (1.5)3 = 6.75
At generation n, the size is of the form 2(1.5)2. So if you want it when n = 80, do 2(1.5)80