15. 1,3:(1+1/12)

1) 1 + 1/12 = 12/12 + 1/12 = 13/12

2) 1.3 / 13/12 = 13/10 / 13/12 = 13/10 * 12/13 = 156/130 = 6/5 = 1 1/5