Simple interest on ₹ 1680 for 4 yr at 71/2 per annum

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Simple interest on ₹ 1680 for 4 yr at 7¹/₂ per annum is equal to the simple interest on ₹ 1200 at 7% per annum for a certain period of time. Time period of time is ?

1. 7 yr
2. 6 yr
3. 5¹/₃ yr
4. 7¹/₄ yr

Correct Option: B

Let the required period of time be T yr.
Then, (1680 x 4 x 15) /100 = (1200 x T x 7)/100
T = (1680 x 2 x 12)/(1200 x 7)
= 6 yr
Hence, the required time period is 6 yr.