The peculiar aspect of your question is those gaps between numbers; it is fairly simple to add 523 and 334, but you have 5 23 and 3 34; it is not clear what you are trying to say. Possilby you mean 5 x 23 and 3 x 34. But it's hard to be sure.
12.
Exactly as in the question: 7+5 = 12
6+6=12, easy :D
The sum of 7/15 and 5/12 equals 53/60
it equals 72. just do 12 X 6
it equals 70
12 plus 9 equals 21.
12+12
20
5 plus 25 equals 60. Need to solve for 5 plus 25 equals ?: 2 plus 10 equals 24 is equivalent to 2 × (1 plus 5) equals 2 × (12); and 3 plus 15 equals 36 is equivalent to 3 ×(1 plus 5) equals 3 × (12); thus 5 plus 25 equals ? is equivalent to 5 × (1 plus 5) equals 5 × (12); thus ? = 5 × 12 = 60.
12 + 10 + 11 equals 33
yes. think of 6/12 plus 1/12. that is 7/12!