You may also like

problem icon

Rotating Triangle

What happens to the perimeter of triangle ABC as the two smaller circles change size and roll around inside the bigger circle?

problem icon

Pericut

Two semicircle sit on the diameter of a semicircle centre O of twice their radius. Lines through O divide the perimeter into two parts. What can you say about the lengths of these two parts?

problem icon

Tri-split

A point P is selected anywhere inside an equilateral triangle. What can you say about the sum of the perpendicular distances from P to the sides of the triangle? Can you prove your conjecture?

Weekly Problem 43 - 2007

Stage: 3 and 4 Challenge Level: Challenge Level:1
We cannot remove two touching coins; if you did, any third coin originally making a triangle with then would then be slideable. We are able to remove the centre and corner coins without enabling any of the remaining coins to slide.If we do this, all of the coins touched at least one of the removed coins so we cannot remove any more.

If we remove a coin which is not one of these four coins first, it would be one of a middle pair of an edge. Four coins touch this coin, leaving five possible coins to remvoe. Four of the five coins would lie along an edge of the frame, with two of these forming a triangle with the fifth. Clearly, only one of the three coins which form a triangle may now be removed, together with only one of the other two coins as they are touching. So we conclude that the maximum number of coins which may be removed is four.

This problem is taken from the UKMT Mathematical Challenges.

View the previous week's solution
View the current weekly problem
The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice. More information on many of our other activities can be found here.
Copyright © 1997 - 2012. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project.