Discussion

Explanation:

S.I. = 2P – P = $\frac{P\times R\times 20}{100}$

⇒ R = 5%

Now, 3P - P = $\frac{P\times 5\times T}{100}$

T = 40 years

Alternately,

Let amount P doubles in 20 years. Hence, interest P (= 2P - P) is accumulated in 20 years.

For the same amount to triple, an interest of 2P (= 3P - P) has to be accumulated.

Since, interest accumulated has to double, the time required will also be double.

∴ Time required = 40 years.

Hence, option (a).

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