A formula like this could take countless forms. If you have a graph and plot the points (10, 1.5) and (30, 1.2) then mostly any random squiggle you draw that passes through those two points represents a possible answer.
But what you want is probably a simple linear function, for which there will be just one solution. These functions are characterised by being "straight", by which I mean that since increasing x by 20 decreases y by 0.3, you can extrapolate that to say that when x = 50, y = 0.9. You can also interpolate to find y when x = 25. but it's best to just find the function first.
Linear functions have the form y = mx + c, so you can plug your known values in and get two equations:
1.5 = 10m + c
1.2 = 30m + c
20m = -0.3
m = -0.015
Plugging in everything once more gives c = 1.65, so your function in this case would be y = 1.65 - 0.015x, and x = 25 gives y = 1.275.