Brain Teasers

1. Hourglass puzzle: You have 2 hourglasses, one is a 7-minute one and the second is a 4-minute one. How can you correctly time 9 minutes?
2. Diophantus' life-span: We know little about this Greek mathematician from Alexandria, called the father of algebra, except that he lived around 3rd century A.D. Thanks to an admirer of his, who described his life by means of an algebraic riddle, we know at least something about his life. Diophantus's youth lasted 1/6 of his life. He had his first beard in the next 1/12 of his life. At the end of the following 1/7 of his life Diophantus got married. Five years from then his son was born. His son lived exactly 1/2 of Diophantus's life. Diophantus died 4 years after the death of his son. How long did Diophantus live?

From: brainden.com