<span>A variable always represents <u>an unknown quantity</u>. We have to have some method to solve for an unknown, and we must have something to hold the place of the unknown, so we use a variable.</span>
The variable represents something that is unknown. If I want to know what to subtract from 13 to get 10, I would say 13-X=10 In this problem the Variable is X, but you can use any letter or symbol, I would not use "S", "O", or "l" to represent a variable, because they can look like 5, 0, and 1.
I suppose as many as we care to, but they're all congruent. We're given two angles and an included side. ASA, angle side angle, is a theorem that yields congruent triangles.