Interesting Math for 3-graders I'm supposed to give a 30 minutes math lecture tomorrow at my 3-grade daughter's class. Can you give me some ideas of mathemathical puzzles, riddles, facts etc. that would interest kids at this age?
I'll go first - Gauss' formula for the sum of an arithmetic sequence.

I am bit undecided about the following riddles, whether they are too difficult for 9-year-olds. Here comes anyway. 


*

*A boy was selling eggs (replace eggs with whatever works locally) to people in a building with four floors. The person living in the fourth floor bought half the eggs plus half an egg. The person living in the third floor bought half of the remaining eggs plus half an egg. The same with the persons living the second floor and in the ground floor. After his tour the boy had sold all his eggs. How many did he have in the beginning? (ok, the fraction may be a dealbreaker here - anyway, the idea is to work it out backwards)

*The combined ages (in full years) of a girl and her mother are 43, those of the girl and her grandmother 69, and the ages of the mother and the grandmother add up to 94. How old are they? (Don't use a system of equations, just add them all up, divide by two, and... The method by trial and error is not to be sneered at either.)

*Have them draw a 4x4 checkerboard, and cut out two opposite corners. Can they cover the remaining 14 squares with 7 domino pieces?  Each domino piece is the size of exactly two squares. How to do it, or why cannot we do it all? (a classic - possibly tough to figure out, but easy to grasp).


Hmm. 11-13 years might be closer to the mark with the above. Rewinding my tape further...


*

*They are probably working their way throught multiplication tables. Ask them to calculate 3x3 and 2x4, then 5x5 and 4x6, then 8x8 and 7x9? Is somebody seeing a pattern? Prove it using a square! If somebody has a sparkle in his/her eye, also try 7x7 and 9x5.

