commision

[ASMAA997]

this question is in the world problems book

Sam earned a $2000 , 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?

My way of solving this question is :

AxS=N

first :
as the new is 900 so , the old is 800

So , 800n=2000

the second will be :
900 ( n+1 ) = S

and i stop here , and I couldn't understand the book solution , may anyone explain it for me in details ?

[RonPurewal]

we can't discuss this problem any further until it's posted in accordance with the forum rules.

please post:
• the entire text of the problem, exactly as originally provided
• the title, edition, and page number of the book from which it's taken

also, you mentioned an answer key that "does something different", so, please post the relevant portion of the answer key as well. thanks.