Sam earned

[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]

this folder is for problems from the FREE GMAT Prep software from mba.com.

please re-post your question in the correct folder. thank you.