If two's company, and three's a crowd, then whats four and five ?

Why is a river so rich ?

Find the missing number in the image below.

There is a square of a particular number which when doubled, becomes 7 more than its quarter.

Can you find the number?

I am the largest in my family of fifty.
The youngest of our family lives separately from the rest of the family and so do I.
I am regarded as harsh and tough.
I am so large, you cut me in two equal halves,
each half would still be larger than the second largest member of the family.
Who am I?

What number does the below three word/phrase identify and why?
1. Deck
2. Year
3. Singing in the Rain

I have an eye but still cannot see.

Who am I?

Two girls were born to the same mother, on the same day, at the same time, in the same month and year and yet they're not twins. How can this be ?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

What is the centre of gravity?