A provider orders 90 ml of a 2.5% mannitol solution. How much of a 25% mannitol solution should be added to water to achieve the appropriate strength?
the amount of mannitol remains constant as water is added, so if x ml of 25% solution are used,
.25x = .025*90
x = 9
makes sense that 1/10 as much is used, since it's 10 times as strong.