I had watered down my tea overmuch... so I poured some of yesterday's from the double-strength cup into it.

While I wouldn't spend **too** much time on it, when those algebra students have to do those "mix 100 L of 5 percent ammonia wiht a certain amount of 50% ammonia and get a 20% solution. How much of the 50% do you need?" problems... it reallydoes help IMO to have **some** idea of what you are doing, so that you can understand basics like that you will end up with more actual LIQUID at the end than you started, but the percentage won't add, it'll be in between.