Name one eight-letter word with 'kst' in the middle, 'in' the beginning, 'and' at the end.?

Can you find the missing letter in the sequence?
S - T - I - L - ? - T - F - Y - C

I get hot but I never sweat,
I cook food but I am not a chef,
I have a door but you can't enter me,
I have a light but I am not a lamp.
Who am I?

Quickly Solve this riddle

7 - 4 + 3 * 0 + 1 = ?

Two guards were guarding the camp.Guard-1 was looking towards the south to make sure no threat is coming from the road.Guard-2 was looking at the north to make sure no threat is coming from the top. Suddenly Guard-1 ask the Guard-2 why he is smiling?How Guard-1 knows that Guard-2 is smiling?

I shave every day, but my beard stays the same. What am I?

Nine marbles are arranged in the image below. Can you slide two marbles to form a square?

A river should be crossed by a father, a mother and their two sons and two daughters.
There are some rules that should be followed while crossing the river. There can be only two people in the raft while crossing. The daughters cannot be with their father unless there is the presence of the mother. The sons cannot be with their mothers unless the father is present. Unless the guard is on the board, the criminals cannot be with any of the family members. Only the adults like the father, the guard, and the mother knows to use the raft.

How will they cross the river?

Below, you will find the mathematical proof that 10 equals 9.99999?. But is that possible or there is something wrong about it? Can you find the error?

x = 9.999999...
10x = 99.999999...
10x - x = 90
9x = 90
x = 10

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?