Two mobile phones were purchased at the same price. One was sold at a profit of 20% and the second was sold at a price which was Rs.1520 less than the price at which the first was sold. If the overall profit earned by selling both the mobile phones was 1%, then determine the cost price of one mobile?

Let the cost price of one mobile phone be $x$.

Total cost price of both mobile phones $= 2x$

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The first mobile was sold at a profit of 20%.

Selling price of first mobile $= x + 0.20x$

$= 1.20x$

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The second mobile was sold at Rs. 1520 less than the first mobile.

Selling price of second mobile $= 1.20x - 1520$

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Total selling price of both mobiles $= 1.20x + (1.20x - 1520)$

$= 2.40x - 1520$

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The overall profit earned was 1%.

Total selling price $=$ Total cost price $+ 1\%$ of total cost price

$2.40x - 1520 = 2x + 0.01(2x)$

$2.40x - 1520 = 2x + 0.02x$

$2.40x - 1520 = 2.02x$

$2.40x - 2.02x = 1520$

$0.38x = 1520$

$x = \dfrac{1520}{0.38}$

$x = 4000$

Therefore, the cost price of one mobile phone is Rs. 4000.