MATHCOUNTS Problem of the Week
Margaret and her friends are playing a card game. In a deck of 36 solid colored cards, each card is purple, blue or green. There are twice as many blue cards as purple cards and the number of green cards is equal to half of the combined number of purple and blue cards. How many cards are green?
Of course, you could just solve it with algebra, but I actually found this one easier to draw out with the unit diagram.
If you click on the link above, the 2nd and 3rd parts of the problem are good beginning probability problems. Once you’ve drawn the unit diagram, it actually makes the probability problems very easy as well.