A scientist mixes water (containing no salt) with a solution that contains 40% salt. She wants to obtain 200 ounces of a mixture that is 5% salt. How many ounces of water and how many ounces of the 40% salt solution should she use?

If it's 5 percent salt, 5 percent = 0.05 and 0.05*200=10 (which is 5% of 200), therefore showing us that we need 10 ounces of salt. From a 40% solution (0.4), we need to figure out how many times 0.4 goes into 10, which is 10/0.4=25, showing us that we need 25 ounces of the salt solution. 200-25=175 ounces left for water