I found some nice and challenging riddles and math exercises i'd like to share with you.
Don't answer if you already know the riddle/exercise and don't Google for the answer and post it here
1. Difficulty : 1/5

How many rectangles in a Sudoku grid? (Note that a square is also a rectangle)

2. Difficulty : 1/5

What's so special about this sequence?

8-5-4-9-1-7-6-10-3-2-0

3. Difficulty: 5/5

Note: the figure is not drawn to scale
ABC is isosceles on A and angle BAC = 20 degrees
D is on [AC] and angle CBD = 60 degrees
E is on [AB] and angle BCE = 50 degrees
Calculate with proofs angle BDE
->Don't draw the figure on the scale and measure the angle, this need demonstrations

In 4 and 5 you have to add as much as you want operations to get the result. You may arrange the numbers the way you want. You may not make 1 and 2 a 12 (twelve). You have to use all the numbers each one once.
(example 2 ,2 and 2 to make 6 => 2+2+2=6)

4. Difficulty 3/5

1, 3, 4, 6 to make 24 ; Operations allowed: + - * / ( )

5. Difficulty 2/5

Operations allowed: + - * / and all you can find on a scientific calculator.

1, 1, 1 to make 6
4, 4, 4 to make 6
8, 8, 8 to make 6
9, 9, 9 to make 6