A) 21
B) -3
C) -1
D) 13
There are two series, beginning respectively with 3 and 7. In one 3 is added and in another 2 is subtracted.
The next number is 1 – 2 = -1
1st Series 2nd Series
3 + 3 =6 7 – 2 = 5
6 + 3 = 9 5 – 2 = 3
9 + 3 = 12 3 – 2 = 1
12 + 3 = 15 1 -2 = -1