Problem No. 24. Three friends

Who is lying?

Ann says "Mine watch is showing 7.30." Betty says "Mine and Ann's watches are showing 7.30." Cintia says "Mine or Betty's watches are not showing 7.30." Only one of three friends is lying. Which one could be lying? If you are allowed to chose, which watch will you want to see, to be certain who is lying?

Teacher tip

Use occasionally logic puzzles to exercise students' logical reasoning.

Problem No. 23. 7 families and a river

A river crossing problem!

7 families with a mother, a father and a daughter each, come to a river they need to cross. There is only one boat for two persons. Only fathers know how to sail the boat. A daughter can be alone only with one of her parents. How can these 7 families cross the river under these conditions? How many times will the river be crossed, less or more than 30?

Parent tip

Explore different combinations and have a good time drawing, sketching, writing the solution. You can even make a paper models for each member of the story and solve the problem through play.

Problem No. 21. Missing numbers again

Here is a new brain teaser!

Find the missing numbers, if the tables follow the same rule.

6	17	2		3	21	1
8		22				12
5	13			6	9	4

Teacher tip

Brain teasers can be used as "worm ups" at the beginning of the class.

Problem No. 20 A cup of water

"Please give me a cup of water!"

You have a 7 cup bottle and a 30 cup bottle. How can you measure one cup of water?

Parent tip

You can make your own 7 cup and 30 cup bottles. Take two bottles, measure the wanted amount of water and mark the bottles, very carefully, at their maximal levels, the first one as the 7 cup bottle, and the other one as the 30 cup bottle. Then you can all have fun, you and your children, with pouring the water in and out of the bottles.

Problem No. 16. Points on a plane

7 different colors are used for coloring 30 points on a plane. Prove that, regardless the way you color them, there will always be at least five points colored in the same color.

Parent tip

You can enjoy a fun family time with your children coloring the points in different colors and different ways.

Problem No. 10. Socks in a drawer

You have 7 pairs of black socks in your drawer, among 30 pairs in total. The rest of them are white socks. What is the smallest number of socks you should take out from the drawer (with your eyes closed) to be sure that you have a matching pair of white socks?

Parent tip

You can give this problem during the house chores. It is the fun way to implement the mathematical thinking outside the classroom. Children should know that math is real, fun and useful.

Problem No. 7. Coins on a table

There are 7 coins laid on a table with the heads up, and 30 coins laid with the tails up. Each coin has a head side and a tail side. How can you split the coins in two piles, with your eyes closed, so that the piles have the same number of coins with the tails up?

Teacher tip

Use logic puzzles at your classes to increase the motivation and to exercise the flexibility in mathematical thinking.

Problem No. 5. Missing numbers

Here is a 5 x 5 table filed with numbers according to some rule. What are the missing numbers?

Parent tip

Exploring patterns with your children can be a fun way to spend some quality time together.

Problem No. 2. Book

Ann usually has the following problem at 7.30 in the mornings: She wakes up and wants to continue reading the book that she had read the night before, but the lamp she uses for reading has depleted batteries and she does have neither spare batteries nor another lamp nearby. But, she always manages to solve the problem very quickly, without leaving either the room or the bed, and continues to read the book. How does she do it?

Parent tip

Suitable for family time, especially with younger children to exercise their listening skills.