Math brain teasers and optical illusions
Brain Teasers and Optical Illusions:
7 Gallons of Water: (easy)
You have a 3 gallon jug and a 5 gallon jug. You need to measure out exactly 7 gallons of water. How do you do it?
The Missing Dollar: (medium)
Three men in a cafe order a meal the total cost of which is $15. They each contribute $5. The waiter takes the money to the chef who recognizes the three as friends and asks the waiter to return $5 to the men.
The waiter is not only poor at mathematics
but dishonest and instead of going to the trouble of splitting the $5 between the three he simply gives them $1 each and pockets the remaining $2 for himself.
Now, each of the men effectively paid $4, the total paid is therefore $12. Add the $2 in the waiters pocket and this comes to $14.....where has the other $1 gone from the original $15?
A number with an interesting property: (hard)

When I divide it by 2, the remainder is 1.
When I divide it by 3, the remainder is 2.
When I divide it by 4, the remainder is 3.
When I divide it by 5, the remainder is 4.
When I divide it by 6, the remainder is 5.
When I divide it by 7, the remainder is 6.
When I divide it by 8, the remainder is 7.
When I divide it by 9, the remainder is 8.
When I divide it by 10, the remainder is 9.

It's not a small number, but it's not really big, either.
When I looked for a smaller number with this property I couldn't find one.

Can you find it?
Wavy Lines Illusion:
Are these lines straight and parallel or wavy?
Look at the chart and say the color not the word:
The Color Quiz
Your right brain tried to say the color, but your left brain was reading the word.

What do you see in this image?
What do you see?
Do you see a beautiful woman or a cool jazz horn blower?
Bunny or Duck?
Image Credit: J. Jastrow (1899)
Duck Rabbit Illusion