Chris, Molly, Sara, and Lynn each has some coins. Chris gives Molly $20$ coins, Molly gives Sara $25$ coins, Sara gives Lynn $30$ coins, and Lynn gives Chris $30$ coins. Now, which of the following is correct if they each wants to have the same number of coins as in the beginning? 
Overseas Competition->Knowledge Point->Combinatorics->Logical Reasoning
Chris gives Molly $20$ coins, and Lynn gives Chris $30$ coins. Now, Chris has $10$ more.  Chris gives Molly $20$ coins, and Molly gives Sara $25$ coins. Now, Molly has $5$ less.  Molly gives Sara $25$ coins, and Sara gives Lynn $30$ coins. Now, Sara has $5$ less.  Sara gives Lynn $30$ coins, and Lynn gives Chris $30$ coins. Now, Lynn has the same as beginning.  Thus, Chris can give Molly $5$ coins, and give Sara $5$ coins. 