Tim 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 tim count?

you have 2 equations to work with

1st eq. w + y = 40

2nd eq. w=9y

multiply everything in 1st equation by 9 to get

9w + 9y = 360

rearrange 2nd equation to w-9y=0

now add 9w + 9y = 360 to w-9y = 0

to get 10w=360

dive both sides by 10 to get w=36 white cars