p/q=⅓/2=1/6 (p:q=1:6); p/r=¾/½=3/2 (p:r=3:2).
q/r=(p/r)/(p/q)=(3/2)/(1/6)=9 (q:r=9:1=18:2).
So r:p=2:3 and r:q=2:18, therefore r:p:q=2:3:18, or p:q:r=3:18:2.
CHECK:
If p:q:r=3:18:2, then p:q:r=15:90:10.
Let p=15, q=90, r=10, then p:q=15:90=1:6=⅓:2; p:r=15:10=3:2; q:r=(15/10):(15/90)=90:10=9:1.