There are two ways to solve this.
The first is to work out the proportion between 1/2 and 5/6 and apply to the other ratio.
5/6÷1/2=5/6*2=5/3. To find f divide 3/4÷5/3=3/4*3/5=9/20.
The second way is to work out the proportion between 5/6 and 3/4 and apply it to find f.
3/4÷5/6=3/4*6/5=9/10. f=9/10*1/2=9/20.