70% respond badly:
20 + 30x0 + 1 =?

ONLY FOR GENIUS:
10 + 10x0 + 20x1 + 5x0+ 3
¿Can you solve this?

D + A = 50
A + A + A = 120
D + C = 35
C = ?

Only few can solve it:
11 x 11 = 4
22 x 22 = 16
33 x 33 =?

My age today is three times what it will be three years from now minus three times what my age was three years ago. 
How old am I?

Mary is 3 years old, David is twice her age, when David will have 60
how old Mary will have? 

77x = 189x = 345x 

¿Which is the value of x?

Only for genius:
C + C + C = 30
C + O = 40
C + O + P = 60
P = ?

D + A = 50
A + A + A = 120
D + C = 35
C = ?

81% fails:
98-15x10 + 50 =?

At college, 70% of the students studied Maths, 75% studied English, 85% studied French and 80% studied German.
What percentage at least must have studied all 4?

How many times can you take 6 to 36?