The doctor orders 500ml of 30% alcohol. you have a 50% solution of alcohol and sterile water in stock. how much of each would you use?

To prepare a 500mL solution of 30% alcohol, you will need to dilute the alcohol you currently possess. 

What you have: 
50% alcohol
sterile water

500mL*.30 = 150mL of the solution should be pure alcohol. Knowing you already have a 50% concentration, just prepare 300mL of 50% alcohol and 200mL of sterile water. Mix the two and you'll get a 500mL solution of 30% alcohol.