What English word has three consecutive double letters?

How can you make the following equation true by drawing only one straight line: 5+5+5=550 ?

There is a shop where written:
Buy 1 for $1
10 for $2
100 for $3
I needed 999 and still only paid $3. How could this be financially viable for the shop-keeper?

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

What is full of holes but still holds water?

How many letters are in the alphabet?

They do not possess flesh or feathers or scales or bone
But still, they are blessed with fingers and thumbs of their own.

Which is Odd One Out and why?

4377

3954

9862

8454

9831

Until it is measured, not known. Yet how you miss it when it has gone. What is it?

A famous swimmer can swim downstream in a lake in exactly 40 minutes with the lake current.
He can swim upstream in that lake in exactly 60 minutes against the lake current.
The length of the lake is 2 km.

How long he can cover the distance of one side at a still lake with no current?