Q.Which is better investment, 20% stock at ₹ 140 or 10% stock at ₹ 70 ?

Rate of return $= \dfrac{\text{annual income}}{\text{amount invested}}\times100$. Both stocks give $\dfrac{100}{7}\approx 14.29\%$, so neither is better — they are equally good.

Assumption. Take the face value of each stock as ₹100 (standard convention). Then a "20% stock" pays ₹20 income per ₹100 face value, and a "10% stock" pays ₹10.

Step 1 — 20% stock at ₹140. Investing ₹140 buys ₹100 face value, earning ₹20 per year.

$\text{Return} = \frac{20}{140}\times100 = \frac{100}{7} \approx 14.29\%.$

Step 2 — 10% stock at ₹70. Investing ₹70 buys ₹100 face value, earning ₹10 per year. …