The square root of number 121 is "11". What is the square root of the number "12345678987654321."?

Prove that twice of ten is the same as twice of eleven.

He digs out tiny caves and stores gold and silver in them. He also built bridges of silver and made crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs his help, yet many people are afraid to let him help them. Who is he?

You have $100 with you and you have to buy 100 balls with it. 100 is the exact figure and you can't go below or above the numbers and you have to use the entire $100. If there is no kind of tax applied how many of each of the following balls will you be able to buy:

Green Balls costing $6
Yellow Balls costing $3
Black Balls costing $0.10

Now, how many of each must you buy to fulfil the condition given?

As a whole I am both
safe and secure.

Behead me and I become
a place of meeting.

Behead me again and I am
the partner of ready.

Restore me, and I become
the domain of beasts.

What am I?

A finger goes in me. You fiddle with me when you're bored. The best man always has me first. What am I?

Solve below popular number sequence
314 159 265 358 979 323 846 ?

Find the Odd One Out?
84129
32418
47632
36119
67626
72927

Count the number of triangles in the given picture.

Prove that the below maths equation is true.

8 + 8 = 91