If we assume your equation is ...
... y = k/x . . . . . . the equation of inverse variation
the value of k is found from any ordered pair of values.
... x·y = k . . . . . . multiply the above equation by x
... 4×12.5 = 5×10 = 10×5 = 20×2.5 = k = 50
The constant in the above equation is 50.