On number six seven eight nine 10 11 12 13 14 15

Easiest ways to solve them are to take the denominator and numerator and and divide the one you are asked to use to answer and divide and see if it works on both. 
  Example: first     105 / 15 = 7 )  Then: 105 / 7 = 15
                             119              )             119 / 7 = 17

Hope I helped. :D