Which of the following numbers will replace the question mark (?) in the given series?
6, 10, 15, 21, ?, 36, 45

6, 10, 15, 21, ?, 36, 45
Logic: series is increasing with the number of 4, 5, 6…..
6+4 =10
10+5= 15
15+6= 21
21+7= 28
28+8= 36
36+9= 45

So, next number is increased with the number of 7. and number is (21 + 7) = 28

Thus, the correct option is: (c).