April 2009 question 8

This is on day 1 of 3 tutorial handout.

For Part ii b) I understand that the numerator is the possible number of meetings of infected/unifected cats but I don't understand why we then divide by 90 to find the transition rate.

Is it to do with the 50% chance of transfer?

Any help appreciated

 

Yes!

 

So where does the 90 come from? Why isn't it just half?

 

1/90 is a half of 1/45!

 

The tutorial handout made no mention of where the 90, nor 45 came from which is why I was so confused!

I've now looked at the examiners report and it is much clearer how the answer was achieved.