I have a bag of marbles, where each marble is pink or yellow. The ratio of pink marbles to yellow marbles is 3:5 If there are total of 72 marbles in the bag, how many are pink?
+590 This might be a horrible explanation, but I'll try. 
First, we know that the ratio is 3:5.
You can also think of this ratio as 3x:5x.
3x+5x=8x. Make this equation: 8x = 72(total).
x=9.
3x would be the part that represents the pink marbles, so plug in 9 for x.
3(9)=27.
There are 27 pink marbles.
