Why is a river so rich ?

How many triangles are there in the picture below?

There are 100 bulbs in a room. 100 strangers have been accumulated in the adjacent room. The first one goes and lights up every bulb. The second one goes and switches off all the even-numbered bulbs - second, fourth, sixth... and so on. The third one goes and reverses the current position of every third bulb (third, sixth, ninth? and so on.) i.e. if the bulb is lit, he switches it off and if the bulb is off, he switches it on. All the 100 strangers progress similarly.

After the last person has done what he wanted, which bulbs will be lit and which ones will be switched off?

What is the next number in the series?

15 29 56 108 208 ?

Can you make a six-letter word using the letters N, A and B ?

Why Aeroplane windows are round?

Can you find the least possible number such that

If the number is divided by 3, it gives the remainder of 1;
If the number is divided by 4, it gives the remainder of 2;
If the number is divided by 5, it gives the remainder of 3;
If the number is divided by 6, it gives the remainder of 4.

Which three-digit number, made of consecutive digits, like 567, is 2 less than a cube and 2 more than a square?

John enters a small room. The Door Closes.
When the door opens, John is in a larger room.

Explain?

How many words can a pen with half refill write?