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.
