Let speed of boat in still water = $$x$$ km/hr and speed of current = $$y$$ km/hr According to ques, => $$frac{12}{x+y}+frac{8}{x-y}=7$$ ----------------(i) and $$frac{18}{x+y}+frac{9}{x-y}=9$$ ----------------(ii) Applying the operation : $$3 imes(i)-2 imes(ii)$$ => $$frac{24}{x-y}-frac{18}{x-y}=21-18$$ => $$frac{6}{x-y}=3$$ => $$x-y=frac{6}{3}=2$$ --------------(iii) Substituting it in equation (i), => $$frac{12}{x+y}+frac{8}{2}=7$$ => $$frac{12}{x+y}=7-4=3$$ => $$x+y=frac{12}{3}=4$$ ------------(iv) Now, adding equations (iii) and (iv), we get : => $$2x=2+4=6$$ => $$x=frac{6}{2}=3$$ => Ans - (D)