A cell phone is available for Rs.600 or for Rs.300 cash down payment together with Rs.360 to be paid after two months. Find the rate of interest charged under this scheme.

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The cash price of the cell phone is Rs.600.
Under the scheme, the buyer pays Rs.300 immediately and Rs.360 after 2 months.
Total payment under the scheme = 300 + 360 = Rs.660.
The extra amount paid over the cash price = 660 - 600 = Rs.60.
This Rs.60 is the interest for 2 months on Rs.300 (the down payment).
To find the rate of interest per annum:
Interest (I) = Rs.60
Principal (P) = Rs.300
Time (T) = 2 months = 2/12 years = 1/6 years
Using simple interest formula: I = P * R * T / 100
=> 60 = 300 * R * (1/6) / 100
=> 60 = (300 * R) / 600
=> 60 = 0.5 * R
=> R = 60 / 0.5 = 120%
But this is the rate for 2 months, so annual rate = 120% * (12/2) = 120% * 6 = 720% which is not possible.
Rechecking the calculation:
Actually, the interest is Rs.60 for 2 months on Rs.300.
So, rate for 2 months = (60 / 300) * 100 = 20%
Annual rate = 20% * 6 = 120%
This matches option 3.
Therefore, the rate of interest charged under this scheme is 120% per annum.

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