A clock is sold for ₹550 cash or in the instalment scheme, for ₹250 cash down payment and ₹310 after one month. Find the rate of interest charged in the instalment scheme.

The correct option is 40%.

Step-by-step Explanation:

1. Determine the Principal for the instalment:
Cash price of the clock = ₹550
Cash down payment = ₹250
Remaining balance (Principal amount to be paid in instalment) = ₹550 - ₹250 = ₹300

2. Determine the Amount paid after 1 month:
Instalment paid after 1 month = ₹310

3. Calculate the Interest charged:
Interest paid for 1 month = Instalment amount - Remaining balance
Interest = ₹310 - ₹300 = ₹10

4. Calculate the Annual Rate of Interest:
The principal for 1 month is ₹300, and the interest on it for 1 month (which is ¹/₁₂ of a year) is ₹10.
Using the Simple Interest formula:

$\text{Simple Interest}=\frac{P\times R\times T}{100}$

Substitute the given values into the formula:

$10=\frac{300\times R\times \frac{1}{12}}{100}$

Simplifying the equation:

$10=\frac{3\times R}{12}$

$10=\frac{R}{4}$

$R=10\times 4=40\%$

Thus, the rate of interest charged in the instalment scheme is 40% per annum.