**I don't know how to figure this out. ****I think... but I'm not sure... let me give you the problem:****How would you prepare 1000ml of a 5% methenamine solution using a 20% methenamine solution? ****I thought about trying the equation with the C1V1 = C2V2. Would that make sense? **

Huh?

If you have something that's 20% methenamine and 80% water, and you need something that's 5% methenamine, well, add more water.

If you had pure methenamine and needed 5% solution, it'd be simpler--50mL pure, 950mL water.

As is, you have 1/5th as much methenamine in your methenamine solution as in a bunch of pure methenamine, so you need 5 times as much solution. 250mL 20% solution, 750mL water.

250ml 20% solution, 950*I am not yet born, console me.I fear that the human race may with tall walls wall me, with strong drugs dope me, with wise lies lure me, on black racks rack me, in blood-baths roll me.*