Rafael counted a total of 40 white cars and yellow cars. there were 9 times as many white cars as yellow cars. how many white cars did rafael count

To work out this question, you need to think of what ratio there is of white cars to yellow cars.
You know that there are 9 times as many white cars as yellow cars.
This means for every white car you have, you have 9 yellow. 1:9.
So for every 10 cars we have (9+1) we have 1 white car and 9 yellow cars.
Next, we take the total number of cars (40) and divide it by the total number of white+yellow cars we have (10)
40 / 10 = 4.
Now, we times 4 by the numbers in the ratio.
1 x 4 = 4.
9 x 4 = 36.
We can double check this by doing 36 / 4 = 9. 

We know that Rafael counted 4 white cars. 

40=w+y
w=y(9)
36=4(9)
Rafael saw 36 white cars and 4 yellow cars