Substitute a number for each variable
Ex.
2x x=5
(2)(5)
=10
13/2 = 6.5
2/13 = 015
6.5-0.15 = 6.35
If his water to cement ratio is 20 to 30, this means that if he uses 20 liters of water, he'd use 30 liters of cement.
The total mixture would then be 20+30=50 liters.
and the water would be 20/50=0.4 or 40% of the total mixture.
