How many pounds of water must be added to 40 pounds of a 10% boric acid to reduce it to a 4% solution?
Think about it like this. Your current solution is 10 parts boric acid and 90 parts water.
You need to figure out how much water you must add in order to reduce this ratio to 4 parts boric acid and 96 parts water.
The important thing to keep in mind here is that the amount of boric acid remains unchanged because you're only adding water to the initial solution - you're essentially diluting this solution.
So, the initial solution contains
#40color(red)(cancel(color(black)("lbs. solution"))) * "10 lbs. boric acid"/(100color(red)(cancel(color(black)("lbs. solution")))) = "4 lbs. boric acid"#
Let's say that
#(4color(red)(cancel(color(black)("lbs."))))/((40 + x)color(red)(cancel(color(black)("lbs.")))) xx 100 = 4%#
Solve this equation for
#4 * (40 + x) = 400#
#160 + 4x = 400#
#4x = 240 implies x = 240/4 = "60 lbs."#
Therefore, if you add