Question 297219
Check your equation. 
At time t=0 hr, {{{N=N0=16}}}.
At time t=1 hr, {{{N=16+2(16)=48}}}.
What function are you using to model? Since the growth rate is proportional to the amount of current value, the mathematical model to use would be the exponential function.
An exponential function would be look like,
{{{N=N0*e^(kT)}}}
You could then use {{{N(1)}}} to find {{{k}}}. 
Since you already know {{{N0}}}, you could plug in {{{T=43}}} to find the critical value. 
Hopefully that steers you in the right direction. 
Re-post if you need more help.