Can you replace the question mark with the correct letter?

6 + 6 + 6 = N
7 + 7 + 7 = E
8 + 8 + 8 = R
0 + 0 + 0 = ?

If an electric train is travelling south, then which way is the smoke going?

A few friends are enjoying their sea voyage in a boat full of apples. On the way, they felt hungry and thus decided to eat the apples. Together, they ate two dozen of apples. When they have eaten the apples, will there be any change in the water level?

Two in a corner, one in a room, zero in a house, but one in a shelter. What am I?

81 x 9 = 801. What must you do to make the this equation true?

If two's company, and three's a crowd, then whats four and five ?

Find three numbers such that When we multiply three numbers, we will get the prime numbers. The difference between the second and the first number is equal to the third and second.

In the picture that is attached with this question, you can find a square which comprises of four little squares inside it. Consider this square to be made with matchsticks. You have to remove two matchsticks such that only two squares remain instead of five.How will you do it ?

Can you decipher these quotes by Albert Einstein?
Blf xzm mvevi hloev z kilyovn lm gsv ovevo lm dsrxs rg dzh xivzgvw.

Be active and find the killer in the picture Riddle below.