Azoo measured the wingspan, in millimeters, of 9 butterflies.
55 50 54 57 50 55 53 52 55
what is the median length of the wingspans?
50 mm
54 mm
55 mm
57 mm

The median is the middle of your data when it is arranged in order.

Since you have 9 data points, the median will be the 5th value.
Your data in numerical order would be:
50 50 52 53 54 55 55 55 57

Therefore, the median is 54mm

54mm is the answer you're looking for. 
I hope I helped! ;)