Instructions: DO NOT USE CALCULATOR. Figures are not drawn to scale. Bashir bought an ordinary mobile and a cover for Tk. 2000. Malek bought a smart phone and the same cover for Tk. 2000 more than what Bashir had paid. The price of the cover was one tenth of the combined cost of the two mobiles. What was the cost of the ordinary mobile?

Explanation: (C) Let O = cost of ordinary mobile, S = cost of smart phone, C = cost of cover. (i) \(O + C = 2000\). (ii) \(S + C = 2000 + 2000 = 4000\). (iii) \(C = \frac{O + S}{10} \implies 10C = O + S\). Add (i) and (ii): \((O + C) + (S + C) = 2000 + 4000 \implies O + S + 2C = 6000\). Substitute (iii) into the combined equation: \(10C + 2C = 6000 \implies 12C = 6000 \implies C = 500\). Substitute C into (i): \(O + 500 = 2000 \implies O = 1500\).