Walter is mixing cement for his new patio. He knows he need to use a water-to-cement ratio of 20 to 30. What percent of his total mixture is water

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.