Radha deposited ₹ 400 per month in a recurring deposit account for 18 months. The qualifying sum of money for the calculation of interest is :

1. ₹ 3600

2. ₹ 7200

3. ₹ 68,400

4. ₹ 1,36,800

Since, Radha deposits ₹ 400 per month in a recurring deposit account for 18 months, thus the amount deposited in first month will earn interest for 18 months, the amount deposited in second month will earn interest for 17 months and so on.

$\text{Qualifying sum }= ₹ 400 \times (18 + 17 + 16 + …….. + 1) \\[1em] = ₹ 400 \times \dfrac{18(18 + 1)}{2} \\[1em] = ₹ 400 \times 9 \times 19 \\[1em] = ₹ 68,400.$

Hence, Option 3 is the correct option.