I enjoy tea, especially green varieties, which taste best when infused at less-than-boiling temperatures. The problem is that my electric water kettle can reach only one temperature reliably: a full-boiling 212 degF. To infuse my tea, then, I have devised a simple, reliable way of heating water to other temperatures.
I start by boiling three cups of water in the kettle. (I know that I need at least that much to warm my cup and infuse my tea.) Then I cool the boiling water to the desired temperature by adding just the right amount of tap water.
The trick, of course, is determining the right amount of tap water to add. In my house the tap water is about 80 degF. Thus to infuse at a green-tea-friendly 180 degF, I must solve the following equation:
3 cups · 212 degF + x cups · 80 degF = (3 + x) cups · 180 degF
Solving, I get x = 0.96, and so I draw just shy of a cup from the tap.
This boil-and-cool method is easy and effective. And it costs less than buying a temperature-adjustable water kettle, which would probably be inaccurate anyway.
Just one more example of how math makes life better.