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Let f(x) be ax+b .. since f(0) is 4 hence b is 4.. now g(x) is inverse of f(x) hence g(x)=(x-4)/a .. also g(5)=10.. hence a=1/10.. hence f(x) becomes x/10 +4.. f(50)= 50/10 +4 i.e;9...... Please approve of you got it..