Non-linear interpolation formula

I have been trying to do this for a while but without success. There is a formula somewhere (using a power law relationship or something) but I have not been able to do it.

I basically need to find a corresponding value (column B) based on an input in column A.

If the inputted value (in column A) is 15, then the output in column B would be 3.75.

Any way of creating such a formula in Excel? Columns A & B hold a non-linear relationship.

Any assistance would be hugely appreciated.

Thanks.