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Option 1 : 7 years

GIVEN:

Principal = Rs. 75000

SI = Rs. 87750

CONCEPT:

Simple interest concept.

Cumulative rate if the rate of interest is ‘R1’ for ‘T1’ years and ‘R2’ for ‘T2’ years = R1T1 + R2T2

FORMULA USED:

SI = PRT/100

Where P = Principal, R = rate of interest, T = Time

CALCULATION:

Suppose the rate was 15% for ‘t’ years.

So,

Cumulative rate = 15 × t + 18 × 4 = (15t + 72)%

Now,

/100 = 87750

⇒ (15t + 72) = 117

⇒ 15t = 45

⇒ t = 3

Hence,

Total time of investment = 3 + 4 = 7 years

∴ the time for which the sum is invested for the varying rate of Interest = 7 years.

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