You can actually type the math into DDG and it will do it for you. Just search for “1.2+5.6+10” (I’m not sure how DDG handles the comma versus period but I would expect that it would adapt to your location)

No clue where it came from. I’ve been using this for about four years now.

It lasts forever with regular maintenance. I moved into a house with a slate roof that’s 96 years old. We just have a roofer come out and replace a few cracked tiles every year. Not necessarily any cheaper but the cost is spread out and it seems less wasteful.