What word is pronounced the same if you take away four of its five letters?

How can you make number one disappear by adding something to it?

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?

I am a prime number.
The double of myself is equal to the square of me.

which number am I?

Place a mathematical symbol between the numerals 5 and 9 in such a way that the resulting number is greater than 5 but smaller than 9.

Can you fill the blank with the correct number?

6 30 870 756030 _____

In an Art Gallary, When the clock hit 11 am, Artist hung the painting number 30. When it struck 4 pm, he hung painting number 240 and when it struck 7:30 pm, he hung painting number 315.

Can you determine, which painting number will he hang when the time is 9:20 pm?

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

Forwards I am heavy, backward I am not. What am I?

What 4-letter word can be written forward, backward or upside down, and can still be read from left to right?