Tying the rope
What is the probability that two pieces of rope are tied together, when Pat ties two loose ends together?
Age
14 to 16
Challenge level
Problem
Sam is holding two lengths of rope by their mid-points.
Pat chooses two of the loose ends at random and ties them together.
What is the probability that both pieces of rope are tied together?
Student Solutions
When Pat picks up one end, there are three possible ends left that she can choose.
One of these will be the other end of the same rope and will produce a loop.
Either of the other two will produce the desired result.
Therefore the probability that Sam is left holding one piece of rope is $\frac{2}{3}$.
Alternative method:
There are six different possible ways of choosing two of the loose ends:
Imagine calling the ends of one rope A and B, and the ends of the other rope X and Y.
These are all the different possible pairings:
A & B
A & X
A & Y
B & X
B & Y
X & Y
A & X
A & Y
B & X
B & Y
X & Y
Four of these pairings lead to the ropes being tied together:
A & X
A & Y
B & X
B & Y
A & Y
B & X
B & Y
Therefore the probability that Sam is left holding one piece of rope is $\frac{4}{6}=\frac{2}{3}$.