A bag contains yellow, green, blue, and white marbles. The ratio of yellow, green, blue, white marbles is $4:1:2:3$. If the bag contains $20$ marbles, then how many blue marbles are there?
2 / ( 4 + 1 + 2 + 3) * 20 =
2/ 10 * 20 =
40 / 10 = 4 blue
+1725 
