math
posted by shank on .
a bag of marbles contains 5 red,7 green ,and some blue.if 1/3 of the marbles in the bag are blue,how many blue marbles are in the bag?

If there are only red, green and blue marbles in the bag, then given that 1/3 are blue, then red and green occupy 11/3=2/3 of the bag.
Use direct proportion:
2/3 bag : 5+7=12
1/3 bag : X
Cross multiply:
x=(1/3)*12÷(2/3)
=12/2
= 6 (blue marbles)