A given object takes n times more time to slide down 45° rough inclined plane as it takes to slide down a perfectly smooth 45° incline. The coefficient of kinetic friction between the object and the incline is

From s = ut+12at2,t=2saFor smooth plane a = g sin θ            For rough plane, a' = g(sin θ - μ cos θ)t' = nt⇒ 2sg(sinθ−μcosθ)=n2sgsinθn2g(sin θ -  μcos θ) = g sin q            When θ = 45°, sin θ = cos θ= 1/2Solving, we get μ=1−1n2