The average of 25 numbers is 13. If the average of the first 13 numbers is 11.5 and the average of the last 13 numbers is 14.5, then the middle number is:

17 19 13 21

13

Sum of 25 numbers = 25 x 13 = 325 Sum of 1st 13 numbers = 13 x 11.5 = 149.5 Sum of last 13 numbers = 13 x 14.5 = 188.5 Middle number is = ( 149.5 + 188.5 ) - 325 = 13