Question 746660
The probability  is made up of the number 
of mice infected/total number of mice.
The fraction 7/12 implies the total
mice = 12 but the numerator 7 does not
work.
So we assume the fraction has been simplified
If we double 7 = 14
this gives us 14/ 10 + 14 = 14/24 which simplifies
down to 7/12 which is whatwe require.
The correct answer therefore is b) 14