Thirty pounds of a salt-water solution contains fifteen percent salt. how much water must be added to weaken the solution to ten percent salt?

15% of 30 pounds is = 0.15 x 30 = 4.5 pounds of salt

So 10% of the solution has to equal 4.5 pounds of salt, so multiply that by 10 for 100% = 45 pounds

So initially you had a 30 pounds solution, so to get to 45 pounds you added 15 pounds of water