Insert the missing number.
3, 4, 6, 8, 9, 12, 12, 16, 15, (....) Select
A) 18
B) 19
C) 20
D) 25
E) 22
Explanation:   There are two series, beginning respectively with 3 and 4. In one 3 is added and in another 4 is added.   The next number is  16 +4= 
<gadget id="calculator"> 16 +4</gadget>
<output>20</output>
20.   Answer: 20) 20
<result>C</result>
Insert the missing number.
3, 4, 6, 8, 9, 12, 12, 16, 15, (....)