Question

Assuming you start with just one microbe that divides every 30 minutes, how many microbes would you have after 8 hours?

Answer 1

1/2^16 or 0.0000152588 microbes

Explanation:

Assuming you start with just one microbe that divides every 30 minutes.

This means that the half life of the microbe is 30 minutes or 0.5hours

The interpretation of this half life is that after every half-life, which in this case is 30 minutes, half of the microbe will be gone, and half will remain.

It follows that after another half hour the amount remaining will be

1/2 of 1/2= 1/4 microbes

Thus after 8 hours, there would have been (8/0.5)=16 half lives.

Therefore the amount of microbes remaining will be 1/2^16 of 1 = 0.0625

Alternatively, we could solve the differential equation

dM/dt=kM, where dM/dt is the rate of decay, and M is the amount at any time t, k is the decay constant

Solution of this first order differential equation by separating the variables and integrating yields {dM/M={kt+c, lnM=kt+c, and ......

M=Moexp(-kt)

The initial value Mo=1, when t=0, and given value M=0.5, t=0.5h yields the value of k as follows

0.5=exp(-k*0.5)

ln(0.5)=-k*0.5

k=1.386

After any time time, thus the given expression holds

M=exp(-1.386t)

Thus after 8 hours, the microbes remaining will be

M=exp(-1.386t)=exp(-1.386*8)=0.000152588 microbes.

Answer 2

Answer: 1/16,384 microbes

Explanation:

Since the microbes divides every 30minutes, if the microbes divides by half every 30minutes, then after an hour it will divides by another half that is (1/2)of 1/2 to give 1/4.

Subsequently after another hour, the microbes will reduces to 1/4 of its remains (i.e 1/4) to give 1/16.

Since the denominator is increasing geometrically each hour.

Looking at the trend;

After 1hr = 1/4 of the original microbe = 1/4

After 2 hrs = 1/4 of the remaining microbe = 1/4 of 1/4 = 1/16

Generalizing, we will let the number of hours be n

According to the progression, 1, 1/4,1/16,...n

Since its a Geometric Progression, nth term = ar^n-1

Where a is the first term = 1

r is the common ratio = 1/4

n is the number of hours of decay

After 8hours, the microbes will have been divided by

(1)(1/4)^8-1

= (1/4)^7

= 1/16,384 microbes