Question:(adsbygoogle = window.adsbygoogle || []).push({});

The number of bacteria present in a culture at time t hours is modelled by the

continuous variable N and the relationship

N = 2000e^(kt)

where k is a constant.

Given that when t = 3, N = 18 000, find

(a) the value of k to 3 significant figures

(b) how long it takes for the number of bacteria present to double, giving your

answer to the nearest minute

k is found to be 0.73241 to 5s.f

For the next part, i simply let N=18000*2=36000, and solved to find t=3.94 therefore it takes 3.94-3 = 0.94 hours for the number to double. This obviously yields the correct answer, however the markscheme let N=4000 subsequently solved to give 0.94 immediately (without having to subtract 3). Can someone please explain the logic behind the markschemes method?

Thanks

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Simple exponential problem

**Physics Forums | Science Articles, Homework Help, Discussion**