Tuesday, 12 January 2016

mathematics - Averaging numbers on the blackboard



Aatif sees the numbers 1,2,3,....,2016 written on the blackboard. In a move Aatif can pick any two numbers on the blackboard, erase them and write instead once their average. As an example, the numbers 1 and 8 may be replaced by 412, and the numbers 2 and 10 may be replaced by 6.


After 2015 moves the blackboard only contains a single number. Can Aatif make his moves so that the final number is 2?



Answer




Yes



First choose 2014 and 2016. Average = 2015. Now take the 2015s. Their average is 2015.


Now choose 2015 and 2013. Average = 2014.


Choose 2014 and 2012. Average = 2013.


Note that we can keep on continuing this approach and end up with a situation like 1, 2 and 4 in the end.



From here, choose 2 and 4. Average = 3. Average of 1 and 3 is 2.


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