Which number should replace the question mark?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

Readers with the sharpest vision can find the eagle in the mountains in 6 seconds. Are you one of them? Test your observation skills now!

In the Chess Board picture below white army is arranged. You need to add a black army on the board such that no piece is under any threat.
Note: Army comprised of 1 king, 1 queen, 2 rooks, 2 bishops, 2 knights, and 8 pawns.

In a school, 280 students studied chemistry, 254 students studied physics and 280 students studied biology.

97 students studied chemistry as well as physics, 138 students studied physics as well as biology and 152 students studied chemistry as well as biology.

73 students studied all the three branches.

Can you find out the total number of students?

Solve the puzzle by telling what does below picture means.

There is one thing that goes round the house and also inside the house but never even share a brief touch. What is it?

I have Rs.50 and I spend it in the way as below.

It is raining today at 11:59 am. What is the probability of sunny weather after 72 hours?

Why is it very easy to weigh a fish?