Crossings
2 out of 3
In this problem we are looking at sets of parallel sticks that cross each other. What is the least number of crossings you can make? And the greatest?
Problem
In this problem we are looking at sets of parallel sticks that cross each other.
Image
| Image
|
There are four red sticks which make four crossings.
Still keeping two sets of parallel sticks, this time with seven sticks in total, can you arrange them in another way, to get a different number of crossings?
What is the least number of crossings you can make?
What is the greatest number of crossings you can make?
Can you find all possible numbers of crossings with seven sticks?
What do you need to do to prove that you have them all or how could you show that you have them all?
Can you find the least/greatest number of crossings for ten sticks?
Can you find the least/greatest number of crossings for fifteen sticks?
Can you predict the least/greatest number of crossings for fifty sticks?
Can you predict the least/greatest number of crossings for any number of sticks?
Getting Started
Remember that you must have two or more sticks to make a set.
Try starting with a set of $2$. How many sticks would be in the set placed across these $2$? How many crossings would that make?
If you can't have a set of $1$ stick, what number could you try next?
Do you notice anything about the number of sticks in each set and the number of crossings? How do these numbers relate to the total number of sticks?
Student Solutions
Tobi and Charles from Luanda International School both sent in correct solutions to this problem. Some of you made the mistake of thinking that you could have one stick crossing all of the rest, but the question did say that there must be 2 sets of parallel sticks.
Tobi's solution is given below.
10 sticks:
To find the biggest number of crossings, put 5 sticks going down and 5 sticks going across so there will be 25.
To find the smallest number of crossings, put 2 sticks going down and 8 sticks across so there will be 16.
To find the biggest number of crossings for an odd amount, instead of splitting the sticks in half split them into the the closest numbers to half (e.g 7 and 8).
To find the smallest number of crossings do the same as you did before, put 2 going down and the rest going across.
Estimate for 50 sticks:
For the smallest amount put 2 down and the rest across (96).
Estimate for any number:
To find the biggest for any number, split the amount in half and put half across and half down but if its an odd amount split the amount into the 2 numbers closest to half.
To find the smallest amount put 2 down and all the rest across.
Charles explains how to find the greatest number of crossings for an odd number of sticks a different way.
Teachers' Resources
Using NRICH Tasks Richly describes ways in which teachers and learners can work with NRICH tasks in the classroom.
Why do this problem?
Possible approach
Key questions
Possible extension
Possible support
Suggest using sticks or straws to model the questions asked and then sketch what has been done.