The marks of 9 students are 36, 41, 28, 47, $x$, 33, 52, 39 and 44. If the median of all nine marks is 40, what is $x$?

Without $x$ the sorted marks are 28, 33, 36, 39, 41, 44, 47, 52. With $x$ the median is the 5th value. If $x$ ≤ 39 the 5th is 39; if $x$ ≥ 41 it is 41. For a median of 40, $x$ itself must be the 5th value, so $x=40$.