[Iznogood]

If they sold n cars the first day, they sold n+6 the second, n+12 the third, n+18 the fourth etc.

So they sold:
1. day: n
2. day: n+6
3. day: n+12
4. day: n+18
5. day: n+24
6. day: n+30

6+12+18+24+30=90
150-90=60
60/6=10

Edit:
Just for the pure fun of it, let's set up the complete equation (in one dimension). Every paranthesis is the sale for one day. Inside the square paranthesis is the total sale for the previous day:
(n) + ([n]+6) + ([n+6]+6) + ([n+12]+6) + ([n+18]+6) + ([n+24]+6)
= 6n + 90 = 150

6n = 150-90 = 60
n = 10

Edit: I have ignored the colors of the cars