we know that
In an Arithmetic Sequence the difference between one term and the next is a constant
where
a1 is the first term
d is the common difference between terms
n is the number of months (in this problem)
case a) Mandy
------> equation
in this equation the first month is April
case b) Bill
-------> equation
In this equation the first month is May
With the equation and the equation
calculate the attached table
therefore
the answer is
December
