2
Seven of them.
The first two prime numbers add up to another prime number.
You can not add two positive prime numbers and get zero.
3
A product cannot exist of a single number - a product is formed by multiplying two separate numbers.
The human body is an organism formed by adding many organ systems together.
1 set
It is 120 if the digits cannot be repeated.
This is called a Fibonacci series after the Italian mathematician who described it.
9,000,000 if there are no other requirements.
There are 9 x 10 x 10 x 10, or 9000 four-digit number that can be formed if the leading digit cannot be zero.
8*10*10=800