What should come in place of the question mark (?) in the following number series?
15, 12, 17, 10, ?, 8, 25, 6
15 - 3 = 12, 12 + 5 = 17, 17 - 7 = 10, 10 + 11 = 21, 21 - 13 = 8, 8 + 17 = 25, 25 - 19 = 6
Note that 3, 5, 7, 11, 13, 17 and 19 are consecutive prime numbers.