- Think of a number. Double it. Add ten to your answer. Take half of the number you now have. Finally, take away the number you first thought of. Your answer is five, isn’t it? How did we know? Are we mind readers, or is there something else going on?

2. In a little village in Canada everybody speaks French or English. If 80% of the people in the village speak French and 60% speak English, what percentage speak both languages?

3. Can you find the sum 1+2+3+…+98+99+100 quickly and with a minimum of calculation?

4. A clock takes five seconds to strike six o’clock. How long does it take to strike twelve o’clock.

5. A lily in a pond doubles in size every day. It covers the pond completely in thirty days. How many days does it take to cover exactly half of the pond?