Johny bought a book and sold it at a loss of 10%. If he had bought it for 20% less and sold it for Tk 55 more, he would have made a profit of 40%, What was the cost of the book?

Explanation: Let \(x\) be the Cost Price (CP) of the book. First sale: Loss of \(10\%\). Selling Price (SP1) = \(x - 0.10x = 0.9x\). Second scenario: New CP (CP2) = \(x - 0.20x = 0.8x\). New SP (SP2) = \(SP1 + 55 = 0.9x + 55\). Profit of \(40\%\) on CP2: \(SP2 = CP2 \times (1 + 0.40) = 1.40 \times CP2\). Substitute expressions for SP2 and CP2: \(0.9x + 55 = 1.40 (0.8x)\). \(0.9x + 55 = 1.12x\). \(55 = 1.12x - 0.9x\). \(55 = 0.22x\). \(x = 55 / 0.22 = 5500 / 22 = 250\). The cost of the book was \(Tk~250\).