4 A scientist is testing models for the growth and decay of colonies of bacteria.
For a particular colony, which is growing, the model is \(P = A \mathrm { e } ^ { \frac { 1 } { 8 } t }\), where \(P\) is the number of bacteria after a time \(t\) minutes and \(A\) is a constant.
- This growing colony consists initially of 500 bacteria. Calculate the number of bacteria, according to the model, after one hour. Give your answer to the nearest thousand.
- For a second colony, which is decaying, the model is \(Q = 500000 \mathrm { e } ^ { - \frac { 1 } { 8 } t }\), where \(Q\) is the number of bacteria after a time \(t\) minutes.
Initially, the growing colony has 500 bacteria and, at the same time, the decaying colony has 500000 bacteria.
- Find the time at which the populations of the two colonies will be equal, giving your answer to the nearest 0.1 of a minute.
- The population of the growing colony will exceed that of the decaying colony by 45000 bacteria at time \(T\) minutes.
Show that
$$\left( \mathrm { e } ^ { \frac { 1 } { 8 } T } \right) ^ { 2 } - 90 \mathrm { e } ^ { \frac { 1 } { 8 } T } - 1000 = 0$$
and hence find the value of \(T\), giving your answer to one decimal place.
(4 marks)