I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

Find the 8 in the below-given Image.

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

Find the next number in the series below

21 32 54 87 131 ?

In the city of Brain Teasers, 5% of people do not list their phone numbers. Now if we select random 100 people from the phone directory, then how many people selected will have unlisted phone numbers?

There is a jar in which there are two types of candies.
20 blueberries and 16 strawberries. You perform the following steps:
1) You take out two candies.
2) If the two candies are of the same flavour, you add a blueberry one otherwise, you add the strawberry one.

You repeat these two steps till there is just one candy remaining in the jar. Which flavoured candy will be left?

You can hear me, but you can't touch or see me.

What am I?

What goes up but never comes down?

Can you find out which triangle will have a bigger area among the following?
1. A triangle with sides 300, 400 and 500
2. A triangle with sides 300, 400 and 700

Can you think of any three-dimensional shape that comprises of exactly two surfaces?