Answer to Parade Magazine "Ask Marilyn" puzzler.

Help a dumb guy out here. In Sunday’s “Ask Marilyn” column in Parade Magazine I read;

Here’s a puzzler: What does this set of equations describe?
1 + 2 = 3
3 + 4 = 7
5 + 6 = 11
7 + 8 = 3
9 + 10 = 7
11 + 11 = 10
Answer: The arithmetic of an ordinary 12-hour clock.

I don’t get it.

8 hours after 7 PM, it’s 3 AM, right?

if it’s 1, then 2 hours from now it’ll be 3.
If it’s 3, 4 hours from now it’ll be 7.
If it’s 5, 6 hours from now it’ll be 11.
If it’s 7, 8 hours from now it’ll be 3.
If it’s 9, 10 hours from now it’ll be 7
If it’s 11, 11 hours from now it’ll be 10.

7 am + 8 hours = 1500, or 3 PM

If it’s 1:00, add 2 hours, get 3:00.

And so on… once you hit the bigger numbers, it’s 7:00, add 8 hours, it’s 3:00.

ETA: I see that I am still very slow with non-digital clocks…

This is called modular arithmetic, actually a very important part of math
(http://en.wikipedia.org/wiki/Modular_arithmetic)