5 and 7 are what are called mutually exclusive prime numbers. Both numbers are prime - they have no common factors between them except 1. So, 5 and 7 will only have
common multiples when 5 is multiplied by a multiple of 7. Some examples of common multiples, then, are: 5*7 = 35 5*14 = 70 5*21 = 105 ...and so on. So, to find out how many common multiples there are that are ...