Matt gets a $1,000 commission on a big sale.This commission alone raises his aver-age commission by $150.If Matt's new average commission is $400,how many sales has Matt made?

- I found the solution like this:

"s=total number of things sold (before big sale)
x=number of things sold (before big sale)

so s/x = 250
s = 250x

after big sale

(s+1000)/(x+1) = 400

s+1000 = 400x+400

since s = 250x

250x+1000 = 400x+400

150x = 600

x= 4

so after the big commision, sales are x+1 = 4+1 = 5"

I don't understand why it is (x+1), meaning the sales has increased by 1, after the big sale. how come (x+1) !