It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

Can you find a pattern and the missing number in the series given below?

100, 155, 258,? , 584, 819.

Mr Black, Mr Gray, and Mr White are fighting in a truel. They each get a gun and take turns shooting at each other until only one person is left. Mr Black, who hits his shot 1/3 of the time, gets to shoot first. Mr Gray, who hits his shot 2/3 of the time, gets to shoot next, assuming he is still alive. Mr White, who hits his shot all the time, shoots next, assuming he is also alive. The cycle repeats. If you are Mr Black, where should you shoot first for the highest chance of survival?

Suppose you are sitting in an interview and the interviewee asks you an aptitude question.

You have three buckets with a capacity of 4 litres, 8 litres and 10 litres and you have a large tank of water. Now you have to measure 3 litres of water precisely using those buckets. How will you do it?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?

Solve this riddle by finding the odd one out from the rest.

There is so small cabin inside a deep forest.
All the forest burned out to worst possible but the inside of the cabin is still the same.
However all the person inside the cabin die.

How come ?

How many people must be gathered together in a room, before you can be certain that there is a greater than 50/50 chance that at least two of them have the same birthday?

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?

Find the smallest sentence with all the English alphabet.