# Sequences and Series problem

A sales team sells 1200 calculators in its first month of operation. They plan to increase their sales by 150 calculators each month. How many calculators do they plan to sell: a) In the last month of the second year of operation; b) Over the entire two year period?

I have solved part a) and the answer is 4650. However, I am having trouble with part b) as I thought finding the answer to this would require using a rate.

• Well if you managed to get the answer to (a) as $1200 + (24-1)\times 150 =4650$, you can get the amount they plan to sell at each month, and add them up. – Calvin Khor Sep 21 '15 at 5:57

It is the sum of the sequence. Hence,

$n = 24, a = 1200, d = 150$

$S_{n} = \frac{n}{2}(2a +d(n-1))$

$S_{n} = \frac{n}{2}(2a +d(n-1))$

$S_{n} = \frac{24}{2}(2(1200) +150(23) )$

$S_{n} = 70200$