Beat The GMAT a GMAT & MBA community

Sam earned a $2,000 commission on a big sale, raising his average commission by $100. If Sam's new average commission is $900, how many sales has he made?

Let total number of sales prior to the big sale be S and
the average commission prior to the big sale be A
Total commission earned prior to the big sale = A*S
Total commission earned after the big sale = A*S + 2000
Total number of sales including the big sale = S+1

Question can be rephrased to What is the value of S+1 ?

900 = Average commission prior to the big sale + 100 = A+100
Implies A = 800

Average commission after the big sale be A = Total commission earned after the big sale/Total number of sales including the big sale
=> (A*S + 2000) / (S+1) = 900
=> (800S + 2000)/(S+1) = 900
=> 800S + 2000 = 900S + 900
=> 100S = 1100
=> S = 11

S+1 = 12 = Total number of sales he made !

p.s: For quick responses,
Please post PS questions here: http://www.beatthegmat.com/problem-solving-f6.html
Please post DS questions here: http://www.beatthegmat.com/data-sufficiency-f7.html

_________________
Anil Gandham
Welcome to BEATtheGMAT
My Quant Blog | Photography | Getting Started | BTG Community rules | MBA Watch
Check out GMAT Prep Now’s online course at http://www.gmatprepnow.com/

You could also do this using allegations.

New commission = Old + 100 = 900.

Old = 800. Resultant = 900 and Addition is 2000. ( 8 and 20 mixed to give 9)

So, (20-9) / (9-8) = 11/1. ( 11 of 8 and 1 of 20)

So, 11 + 1= 12 commissions.