Wich if the given numbers are composite?15,19,23,31,49,55,59,87,91,97,103,105

Answer 1

Compare to the list of them here:

http://www.math-worksheets.info/Composite_numbers.html

The list goes as far as 100. Beyond that, 103 is prime and 105 has more than two prime factors (5,21,3,35,7, and 15), so they are not composite.