exam math questions

by Brittany
(TX)

Hey guys, I finished the pharm tech program at the end of may and have been studying since then for the certification exam (which i take tomorrow). I somewhat understand the solution dilution and concentration and proportion problems but I have problems when I read the question as to which formula to use? I have a few example questions from the mosby's practice CD if I put them on here could someone show me step by step how to solve? Also are there any hints as to how to figure out which formula to use for a problem?

How many ml's of water must be added to 250ml of a 0.9% w/v stock solution of sodium chloride to prepare 1/2 NS solution?

How many ml's of a D10W solution contain 100g of Dextrose?

What is the % strength of a solution that is made by adding 100ml of water to 600ml of a 25% solution?

How many grams of zinc oxide would contain 10g of zinc oxide?