Number Series Puzzles

1. Which number replaces the question mark ?

    6,9,15,27,?

A:-  51
    
        6
        6+3= 9
        9+6(3*2)=15
        15+12(6*2)=27
        27+24(12*2)=51 .

Another solution:-

        6
        6+6 = 12-3 = 9
        9+9 = 18-3 = 15
        15+15 = 30-3 = 27
        27+27 = 54-3 = 51.

2. Which number replaces the qustion mark ?

     26,28,31,35,?

A:- 40


       26
       26+2=28
       28+3=31
       31+4=35
       35+5=40.


3. Which letter replaces the question mark?

     O,T,T,F,F,S,S,E,?

A:- N


      One
      Two
      Three
      Four
      Five
      Six
      Seven
      Eight
      Nine.