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

15 caves are arranged in a circle at the temple of doom. One of these caves has the treasure of gems and wealth. Each day the treasure keepers can move the treasure to an adjacent cave or can keep it in the same cave. Every day two treasure seekers visit the place and have enough time to enter any two caves of their choice.

How do the treasure seekers ensure that they find the treasure in the minimum number of possible days?

Count the number of triangles in the picture below:

What is the next number in the series?

15 29 56 108 208 ?

Find the next number in the series?
2, 12, 36, 80, 150, 252, 392, _ ?

The captain of a ship is telling you an interesting story and then poses a question. He says, “I have travelled the oceans far and wide. One time, two of my sailors were standing on opposite sides of the ship. One was looking west and the other one east. And at the same time, they could see each other clearly. Can you tell me how that was possible?”

How many words can a pen with half refill write?

When you say goodbye, I am what you need to tie.
I come as a couple and go wherever you run.
Who am I?

If,
9 * 8 * 7 = 65
8 * 7 * 6 = 50
7 * 6 * 5 = 37

Find
6 * 5 * 4 = ?

What always comes into a house through the keyhole?