In a shop, 40% socks are white and rest are black. 60% socks are made of cotton and rest are made of wool. 25% white socks are made of cotton and 20 of the black socks are made of wool. How many black socks are made of cotton?

Explanation: Let the total number of socks be \(20x\). White socks = \(40\%\) of \(20x = 8x\). Black socks = \(60\%\) of \(20x = 12x\). Cotton socks = \(60\%\) of \(20x = 12x\). Wool socks = \(40\%\) of \(20x = 8x\). White Cotton socks = \(25\%\) of \(8x = 2x\). White Wool socks = \(8x - 2x = 6x\). Black Wool socks = 20. Black Cotton socks = \(12x - 20\). Total Wool socks = White Wool + Black Wool. \(8x = 6x + 20\). \(2x = 20 \Rightarrow x = 10\). Number of Black Cotton socks = \(12x - 20 = 12(10) - 20 = 120 - 20 = 100\).