In how many years will the amount of Rs. 1200 double itself under compound interest system of 4% p.a.? (log 2 = 0.30102, log 1.04 = 0.01703)

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RRB ALP Electronics Mechanic 21 Jan 2019 Official Paper (Shift 1)
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  1. 18.5
  2. 17.67
  3. 19.33
  4. 20

Answer (Detailed Solution Below)

Option 2 : 17.67
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Detailed Solution

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Given:

Principal = Rs. 1200, Total amount = Rs. 2400, Rate = 4%

Concept used:

log 2 = 0.3, log 1.04 = 0.017

Formula used:

A = P[1 + (r/100)]t

Calculation:

Let, duration = n years

⇒ 2400 = 1200[1 + (4/100)]n

⇒ 2 = [1 + ((4/100)]n

⇒ 2 =  [104/100]n

 2 =  [1.04]n

Applying log on both sides,

⇒ log 2 = log [1.04]n

⇒ log 2 = n × log [1.04]

⇒ n = log 2/log 1.04 = 17.67

∴ Option 2 is the correct answer.

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