Divide and conquer

Steps

1. Lay out sorted cards face down. Say they run from small to large.
2. Ask the class to find 52 using as few turned cards as possible.
3. Turn over the middle card. If it is too big, throw away the right half. If too small, throw away the left half.
4. Look at the middle of what is left and repeat until you find 52.
5. Count the guesses with the class. Try other targets.
6. Compare with the unsorted search. What changed?

Talk about it

• How many guesses at most for 15 cards?
• What if we doubled the cards?
• What if the number is not there?