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.
100000 - including numbers with leading 0s
Assuming leading zeros are not permitted, then: If repeats are not allowed there are 30 possible numbers. If repeats are allowed there are 60 possible numbers.
It is 120 if the digits cannot be repeated.
9,000,000 if there are no other requirements.
81
100000 - including numbers with leading 0s
You can not add two positive prime numbers and get zero.
Assuming that numbers with a leading zero is not permitted, the answer is 9*9! = 3,265,920. If leading 0s were permitted, the answer would have been 3,628,800
2
A product cannot exist of a single number - a product is formed by multiplying two separate numbers.
Assuming leading zeros are not permitted, then: If repeats are not allowed there are 30 possible numbers. If repeats are allowed there are 60 possible numbers.
1 set
It is 120 if the digits cannot be repeated.
9,000,000 if there are no other requirements.
22, if leading 0s are disallowed. 24 if they are permitted.
81
8*10*10=800