Find the interquartile range 18,20,21,22,24,24,25

You first find the median, which is the central term if the number of terms is odd, or the average of the two central terms if the number of terms is even...

This is an odd number of elements and the center term, the median, is 22

Now you get the "median" from the first term to the median, which is (20+21)/2=20.5  That's the first quartile...

Now you get the "median" from the central term to the last term which is:

(24+24)/2=24.  That's the third quartile...

Now the interquartile range is Q3-Q1 so:

24-20.5=3.5

So the interquartile range is 3.5