What is a composite number between 70 and 80?

All numbers except ##{71,73,79}## are composite numbers between 70 and 80 i.e.

##{70,72,74,75,76,77,78,80}## are composite numbers.

All numbers which are not are composite numbers.

To test whether a number is prime number or not one needs to try dividing the number only by prime numbers up to approximately of this number.

As we have to find composite numbers between ##70## and ##80##, we need to check for prime numbers up to ##9## (as ##9^2=81##) i.e. ##{2,3,5,7}## only.

These numbers do not divide ##{71,73,79}## and hence other than these,remaining numbers are composite.