Question

13, 15, 17, x, 20, 21; The median is 18.

Answer 1

To find the median

 ⇒ must define what is a median

    ⇒Definition of median: middle number of an ordered set

Lets look at the information given

 Set when ordered: 13,15, 17,x,20,21     with median of 18

  ⇒ In this case, there is an even number of numbers

    ⇒ to find the median

       ⇒ must find average of two numbers closest to the middle

             ⇒ which are 'x' and '17' which is equal to the given median 18

Let's set up the equation:

  $\frac{x+17}{2} =18\\x+17=36\\x=19$

Answer: x = 19

Hope that helps!