#### Answer

19

#### Work Step by Step

3,9,15,5,11,17,7,13
the above pattern is as follows:
First numbers is obtained by adding 6 to the previous number(3,9,15)
Subtract 10 from the third number to get the fourth number of the series (15-10) =5
again add 6 to the set of next 3 numbers(5,11,17)
Again subtract 10 from the last number(17 -10) = 7
Again add 6 to get the next 3 numbers (7,13,19)
so the next number is 19.