# Method to predict outcome as percentage of two categories? [duplicate]

In real world example I have elemental composition data for groundwater, which is known to be coming from two sources. I have control data where all predictors are from source A and data where all predictors are from source B. I would like to be able to take an unknown water sample and predict what percentage is source A vs. B based on the elemental composition of that sample.

The problem is not obvious to me because it is not a categorical outcome, but rather the relative contribution of two potential categories - if that makes sense.

I've looked for similar questions but couldn't find one - be gentle! Thanks