C20/25 is a measurement of class strength, pertaining to 25 MPa of compressive strength.

S1 is a consistency class of concrete at the fresh state, and has lower workability than S3.

A superplasticizer can be added to a concrete mix, which raises the water requirement by 20%.

In the Compressive Strength vs. W/C ratio graph, a lower W/C corresponds to a higher Rc [MPa].