A college has thirteen instructors qualified to teach a special computer lab which requires two instructors to be present. How many different pairs of teachers could there be?
a. 110
b. 165
c. 60
d. 55
Please show a bit of work so I can understand. :)

Question

anonymous · Answer

For each of the 13 instructors, how many others are left to pair with them? Then, of all the possible pairs you've found, how can you cut down the duplicates?