Question

How much water should be mixed with 237 ml of​ ammonia, whose strength is​ 100%, in order to create a mixture that is diluted to a​ 75% strength.

Answer 1

Let the amount of water to be mixed in be x mL. If we currently have 237 mL of 100% ammonia, and we are to add x mL of pure water (which has zero ammonia), we can add the total amounts of ammonia in the final mixture, which will have (237 + x) mL of liquid, with 75% ammonia.
(237 mL)(100%) + (x mL)(0%) = (237 + x mL)(75%)
237 = 177.75 + 0.75x
x = 79 mL
Therefore, we would need to add 79 mL of water.