27,28,25,25,23,22,21,? | series Logical reasoning | WBCS

Complete the series by finding the missing number from the given options.

27,28,25,25,23,22,21,?

(a) 20
(b) 21
(c) 19
(d) 18

Correct Answer: c

Solution: 

series 1:
27-2=25
25-2=23
23-2=21

series 2:

28-3=25
25-3=22
22-3=19