maths MEDIAN問

the mean of seven numbers is 20.The mean of the four largest numbers is 23 and the mean of the four smallest numbers is 17.Find the median.
the total sum of the seven number=20*7=140
the total sum of the four largest numbers =23*4=92
the total sum of the four smallest numbers =17*4=68
we know that
the total sum of the four largest numbers +the total sum of the four smallest numbers=the total sum of the seven number+the median
92+68=140+the median
the median=20