Question 745105
 f(x)=4-5x  and  g(x)=4/5-1/5 x  
(fog)(x)=x 
or (fog)(x)=f(g(x))=f(4/5-1/5 x)=4-5(4/5-1/5 x)=4-(4-x)=4-4+x=x proved

and (gof)(x)=x.
(gof)(x)=g(f(x))=g(4-5x)=4/5-1/5(4-5x)=4/5-(4/5-x)=4/5-4/5+x=x