Question

In a box of marbles, the ratio of red marbles to total marbles is 2:5. the ratio of green marbles to total marbles is 3:10. if the marbles that are neither red nor green are blue, how many blue marbles are in the box if there are 40 marbles in the box?

Answer 1

Ratio:

Red : Total 
  2   :    5
 (x2)    (x2)
  4   :   10

Green : Total
  3   :       10

Total of red and green marbles = 4 + 3 = 7

Blue marbles = 10 - 7 = 3

10 parts = 40 marbles
1 part = 40 ÷ 10 = 4
3 parts = 4 x 3 = 12

Answer: There are 12 blue marbles.