Not sure if there should be options listed, but here is an equation he should use:
f(t) = 3*2^t, where t= number of months since first observation.
or
f(t)= 3*2^(t-1), if the moment of observation is t=1
If this isn't an availabe option, plug in a few values for t (number of months) and make sure they match our values
Based on the first equation:
When t = 0 (when Roberto first observes the bacteria):
f(0) = 3*2^0 = 3*1 = 3
f(1)=3*2^1 = 3*2 = 6
f(2)= 3*2^2 = 3*4 = 12
and so on until...
f(43)=3*2^43 = 26,388,279,066,624 That's a lotta bacteria!
