UPSC CAPF AC 2020 Paper

Let the Principal = ₹ P
Rate of interest = r% p.a
we know that, A=P(1+

r

100

)r
∴ Amount in 10 yr =P(1+

r

100

)10
⇒ 3P = P (1+

r

100

)10
⇒ 10√3=P(1+

r

100

) ........(i)
Let the amount will nine times of itself in t yr.
Then, 9P = P (1+

r

100

)r
⇒ 32 = (10√3)t = 3

t

10

[ ∵ from eq (i) ]
On comparing both sides
⇒ 2 =

t

10

∴ t = 20 yr.