TST Lam Dong 2010
Make boys and girls a line. Divide the line by two parts such that number of boys and number of girls are equivalent. Let be number of cases which cannot divide, let be number of cases which can divide by one way. Prove that .
Solution. One way division the line is choosing boys and girls. The line has two part, left part and right part. And two ways division are equivalent if their left part has same of number boys and girls.
With the only way division, (suppose boys and girls in left part), we change a boy in left part with a girl in right part then we attain a impossible way. And we change a girl in left part with a boy in right part then we attain a impossible way too.
So we have correspondence of one-one: the one way division and two impossible way division, this implies . QEA