Annabel has 25 ounces of a 20% boric acid solution which she wishes to dilute to a 10% solution. How much water does she have to add in order to obtain the 10% solution?

This can be solve by establish an equation

let x be the amount of water to be added

y is the amount of the 10 % soultion formed

25 + x = y

then the other equation will be

25 (0.80) + x = 0.90y

solving the equtions simultaneously

x = 25 ounces of water is need to dilute the solution to 10 % boric acid