Question

you have 47 mL of a 2 M stock solution that must be diluted to .5 M how much water should you add

Answer 1

you want to dilute it (2/.5) times, or 4 times. That means one part stock, 3 parts water.

so one part is here 47 ml. YOu add it to 3 times 47ml water.

Your teacher is just too easy. Here is a harder version, expect it on a test:
You have stock soluton of 2M that must be diluted to make 89ml of .5M concentration. How do you do that?

again, diluting (2/.5) or 4 times; one part original stock, three parts water.

one part of stock is 1/4(89ml)=89/4 ml stock.
add to three parts (3/4)(89ml) water. Stir, label, and properly store.