Can you start the fire using two pieces of wood?

How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?

A boy and a girl are sitting on the porch.
"I'm a boy," says the child with black hair.
"I'm a girl," says the child with red hair.
If at least one of them is lying, who is which?

Baseball bat and ball cost $100. If the bat cost $99 more than the ball, what is the cost of each?

How many triangles are there in the picture below?

What is always in front of you but can’t be seen?

Using four sevens (7) and a one (1) create the number 100. Except for the five numerals, you can use the usual mathematical operations (+, -, x, :), root and brackets ()

A 3 digit number is such that it's unit digit is equal to the product of the other two digits which are prime. Also, the difference between it's reverse and itself is 396.

What is the sum of the three digits?

If "P" means "-", "Q" means "/", "R" means "+", and "S" means "*" then find the value of the following:

21 Q 7 R 9 S 10 P 13

The dirtier I get, the whiter I look. Who am I?