if a 0.1mm paper is folded 100 times...

if a 0.1mm paper is folded 100 times then what will be the thickness
of it after 100 foldings?

English sentence with all the letters

Would anyone know the shortest sentence in the English language
that has every letter of the alphabet?

Clock Angles

How many times are the hands of a clock at right angle
in a day?

Water jar puzzle

You have three jars that hold max 19, 13 and 7 liters of water
respectively. 13 and 7 litre jars are full of water and 19 litre jar
is empty (so total 20 litres of water).

You can pour water between jars and I want two jars with 10
litres of water (ie 10, 10, 0 from 0, 13, 7)

Unlock the Safe

There is a safe with a 5 digit number as the key. The 4th
digit is 4 greater than the second digit, while the 3rd digit
is 3 less than the 2nd digit. The 1st digit is thrice the
last digit. There are 3 pairs in the number whose sum is 11.

Find the number.

3x3x3 wooden cube

Imagine a 3x3x3 wooden cube.

How many cuts do we need to break it into 27 1x1x1 cubes?

Crossing the river

Two boys want to cross a river. The only way to get to the other
side is by boat, but that boat can only take one boy at a time.
The boat cannot return on its own, there are no ropes or similar
tricks, yet both boys manage to cross the river using the boat.

How they manage to cross?