a$)$What is counterfactual fairness? Explain with a suitable illustration.

b$)$Consider the problem of predicting house prices using three attributes, namely, x$1$: Number of bedrooms, x$2$: Size of the house $($in hundreds square feet$),$ and x$3$: Age of the house $($in years$).$ Consider machine learning mode F$($x$1,$x$2,$x$3)=5*$x$1+3*$x$2-2*$x$3.$ Compute marginal contribution and SHAP values of each of the attributes for data point $(3,15,2).$