Which tank will fill first?

In case you were starting to feel confident, this one was meant for third graders in Vietnam. The answer is 66, but we don't blame you for scratching your head about how they got there.

There is a unique number which when multiplied by any number from 1 to 6, we will get the new number that contains the same digits only.

Can you find that number?

I cannot talk, but I always reply when spoken to. What am I?

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

You’ll find me on Mercury, Earth, Mars, Jupiter, Saturn, and Uranus. But never Neptune, or Venus. What am I?

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?

Why do we preferably have round manhole covers and not square ones?

What is the next number in this series?

4,12,84,3612....

If,

A + B = C
D - C = A
E - B = C

Based on the above equations, find out the answer for:

D + F = ?