The average of 25 numbers is 18. The average of the first 12 of these numbers is 14 and that of the last 12 of these numbers is 17. The 13th number is:

80 78 74 82

78

Sum of 25 numbers = 25 x 18 = 450 Sum of first 12 numbers = 12 x 14 = 168 Sum of last 12 numbers = 12 x 17 = 204 13th number = 450 - 168 - 204 = 78