Problem
COM-B1-M12-P003 Set 1. Coins
#3
★☆☆☆☆ Level 1 of 5
There are \(9\) coins heads up. In one move exactly two coins are flipped. Can all coins become tails up?
Track the parity of the number of heads.
The number of heads changes by \(-2\), \(0\), or \(2\), so its parity is invariant. It starts at \(9\) and would need to become \(0\). Odd cannot become even, so it is impossible.
Invariant in simple form.