put the numbers in order
12,13,17,21,25,x
(17 + x) / 2 = 18
17 + x = 36
x = 36 - 17
x = 19
lets try it...
12,13,17,19,21,25.....now take the 2 middle numbers, add them, then divide by 2 to get the median
(17 + 19) / 2 = 18
36/2 = 18
18 = 18 (correct)
ur missing number is 19 <===