a/b = c/d => a/c = b/d => d/b = c/a => b/a = d/c
2. x/y= z/t => t/y = z/x => x/z = y/t => y/x = t/z
3. 4/15= 8/30 => 56/12 = 40/4.15 => 4.15/40 = 21/56 => 21/4.15 = 56/40
4. 15/21= 40/56 => 15/40 = 21/56 => 56/21 = 40/15 => 21/15 = 56/40
5. x/7= y/13 => x/y = 7/13 => 13/7 = y/x => 7/x = 13/y
6. m/n= p/q => q/n = p/m => n/m = q/p => q/n = p/m
7. AB/CD= EF/GH => GH/CD = EF/AB => AB/EF = CD/GH => CD/AB = GH/EF
8. MN/PQ= 2/3 => 3/PQ = 2/MN => MN/2 = PQ/3 => PQ/MN = 3/2