Assume your data is fairly linear
and looks like this:
Xs Ys
246.2 40.7
54.6 1.9
146.3 19.0
102.5 8.8
296.6 49.2
205.0 30.6
Then the interpolated value for 120
can be found with this formula:
=FORECAST(120,Ys,Xs) = 13.9