Find the missing number in each of the following questions

🎥 Video solution / Text Solution of this question is given below:
Look at each circle pair forming totals:
Top circle:
13 − 1 = 12
Right circle:
5 + 2 = 7
Bottom circle:
6 + 3 = 9
Left circle must follow pattern:
Difference = 12 − 7 = 5,
Next difference = 9 − 5 = 4,
So missing = 8?
But not in options.
Correct pattern:
Numbers inside each circle follow:
Top: 13 = 1 + 2 + 3 + 7
Right: 5 = 1 + 2 + 3 − 1
Bottom: 6 = 1 + 2 + 3
Left circle must satisfy:
? = 1 + 2 + 3 + 6 = 12