for every 10 white cars a dealer sells he sells 7 silver, 6 blue, 5 red, 4 yellow, 3 green, 2 black,

for every 10 white cars a dealer sells he sells 7 silver, 6 blue, 5 red, 4 yellow, 3 green, 2 black, 2 purple and 1 brown car. If he sells 120 cars how many blue cars?

Take this in blocks, so each block has:
10 white + 7 silver + 6 blue + 5 red + 4 yellow + 3 green + 2 black + 2 purple + 1 brown = 40 cars

Calculate the number of blocks:
120 cars / 40 cars = 3 blocks.

For 120 cars sold, it takes 3 blocks, which means we multiply:
6 blue cars per block * 3 blocks = 18 blue cars