A painter uses 2 tubes of black paint for every 6 tubes of white paint to paint a mural. At that rate, how many tubes of black paint are used if 15 tubes of white paint are used?

For 14 tubes of black paint 42 tubes of white paint are needed.

The to get the extra tube you have to think that half of 2 is 1 and half of 6 is 3.

so you add 3 and 42 and you get 45