How many sets of 2 without duplicates out of these options? So there are twelve signs of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces I want to know how many possible sets of 2 I can make out of these, for example, Aries with Aries, Aries with Taurus, Aries with Gemini, etc, without duplicates. So for example, if I have Aries with Taurus, let's forget about Taurus with Aries now. What's the maths called that represents this and how to use it?

There will be $\binom {12}2 = \frac{12 \times 11}{2} = 66$ pairs of non-matching signs and another $12$ pairs of matching signs. All together, that's $78$ pairs total.

Another way to find the answer is to see that it's $12 + 11 + 10 + \cdots + 2 + 1$ all together.