Friday, October 10, 2014

5 math riddles that will have you scratching your head!

Another Friday (time sure seems to fly sometimes!) and I'm linking up once again with Doodle Bugs Teaching for her Five for Friday.  This week, my theme was math riddles.  On the phone with my dad earlier this week, we reminisced over the math riddles that often came up during long road trips when we were younger, and I knew that was going to be my theme for this week!

Are you ready to get your math game on?

A knight comes to a castle where the princess is trapped. The castle is surrounded by a square moat that is 10 feet wide (it forms a square around the castle). The knight is able to fashion two 9 foot long planks to cross the moat.

How can he do this?

(Answers will all be at the end of the post to give you a chance to try!)

Does an ounce of gold or an ounce of feathers weight more? (Yes, a classic!)

Caleb is walking home. He has $400 he has to bring home to his mom. While he is walking a man stops him and gives him a chance to double his money. The man says "I'll give you $800 if you can roll 1 die and get a 4 or above, you can roll 2 dice and get a 5 or 6 on at least one of them, or you can roll 3 dice and get a 6 on at least on die. If you don't I get your $400."

What does Caleb do to have the best chance of getting home with the money?

Use the numbers 2, 3, 4 and 5 and the symbols + and = to make a true equation.

Note: Each must be used exactly once and no other numbers or symbols can be used.

As I was going to St. Ives,
I met a man with seven wives,
Each wife had seven sacks,
Each sack had seven cats,
Each cat had seven kits:
Kits, cats, sacks, and wives,
How many were there going to St. Ives?

Answers :

1 : The Knight in Shining Armour

He puts one plank across the corner of the square (where two sides meet) and puts the other perpendicular to it to form a 'T' shape in the corner allowing him to cross the moat and save the princess.

2 : The Classic

You would think they both weigh the same, right?  Nope! In fact, the gold weighs more!In avoirdupois weight (feathers are measured in this way), one ounce = 437.5 grains. In troy weight (gold is measured in this way), one ounce = 480 grains.

Now read on to find out why a pound of feathers actually weighs more than a pound of gold : Measuring Gold Weight

3 : Poor Caleb

This is the perfect riddle to introduce probability to your students!  The best way for Caleb to keep his $400 is to not take the bet at all! This gives him a 100 percent chance of getting the money home. 

If he takes the bet with 1 die he has a 50 percent chance of winning. If he takes the bet with 2 dice he has about a 56 percent chance of winning. If he takes the bet with 3 dice he has about a 42 percent chance of winning.

4 : Equation time!


Perfect to introduce students to exponents! haha  

5 : Read the question thoroughly!

I love this one, it really teaches students to read, figure out what the question is asking and then only using the data required!

The answer?  Only 1 is going to St.Ives, me! :)

Want more great math riddles? Check out this website.  These are perfect for your math class - and I bet you'll find that your "weaker" math students are often the ones that can answer these correctly!

Have a fantastic weekend everyone!  Don't forget to head back to Doodle Bugs Teaching for more fantastic Five for Friday posts.