Divide both by their highest common factor (HCF).
78/91 = 6/7
It is: 13/78 = 1/6 simplified
78/61 cannot be simplified further.
78/105 = 26/35
It is: 78/96 = 13/16 simplified
Yes.
78 simplified is 78.
It is: 78/126 = 13/21 simplified
78/100 simplifies to 39/50
12/13
3/5
It is: 78/120 = 13/20 simplified