A good answer might be:

5. At the end of month 4 there were 3 rabbits, 2 of which are mature. The mature rabbits each produce another female resulting in 5 rabbits in month 5.

Rabbits Rule

The updated chart shows the situation.

N	1	2	3	4	5	6	7	8	9	10
Rabbits	1	1	2	3	5

Here is what we have been doing:

rabbits_At_End_Of_Month( N ) = rabbits_At_End_Of_Month( N-1 ) 
                                             + 
                               rabbits_Born_At_End_Of_Month( N-1 )

rabbits_At_End_Of_Month( N ) = rabbits_At_End_Of_Month( N-1 ) 
                                             + 
                               mature_Rabbits_At_End_Of_Month( N-1 )

rabbits_At_End_Of_Month( N ) = rabbits_At_End_Of_Month( N-1 ) 
                                             + 
                               rabbits_At_End_Of_Month( N-2 )

Once you have this rule, to calculate the number of rabbits to put under a month in the chart just add up the entries for the previous two months.

A good answer might be:

Rabbits Rule

QUESTION 13: