Z repaid ₹10,920 after 4 years at 10% per annum simple interest. How much money did he borrow?

Use the simple interest relation:

$A = P \left(1 + \frac{RT}{100}\right)$

Given:
Amount $A = 10920$, Rate $R = 10%$, Time $T = 4$ years

Substitute:

$10920 = P \left(1 + \frac{10 \times 4}{100}\right)$

$10920 = P (1 + 0.4)$

$10920 = 1.4P$

So,

$P = \frac{10920}{1.4} = 7800$

Final Answer: ₹7,800