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A man A borrows Rs 8000 at 12% per annum simple interest and B borrows Rs 9100 at 10% per annum simple interest. In how many years will their amounts of debts be equal?

Answer
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Hint: Assume a variable t which represents the time in years after which the amounts of debts of A and B becomes equal. In simple interest, we have a formula to calculate the amount after t years for a principal amount p at a rate of interest r per annum simple interest. This formula is amount = p+p×r×t100 . Use this formula to get an equation in t which can be solved to get the value of t.

Complete step-by-step answer:
Before proceeding with the question, we must know all the formulas that will be required to solve this question.
For a principal amount Rs p at a rate of interest r per annum simple interest, the amount after t years is given by the formula,
Amount = p+p×r×t100.............(1)
In the question, A borrows Rs 8000 at 12% per annum simple interest and B borrows Rs 9100 at 10% per annum simple interest. We are required to find the number of years after which their amounts become equal.
Let us assume that after t years, their amounts become equal.
Since A borrows Rs 8000 at 12% per annum simple interest, using formula (1), the amount after t years is equal to,
8000+8000×12×t1008000+960t.............(2)
Since B borrows Rs 9100 at 10% per annum simple interest, using formula (2), the amount after t years is equal to,
9100+9100×10×t1009100+910t.............(3)
Since the amounts we got in (2) and (3) are equal, we get,
8000+960t=9100+910t50t=1100t=22
Hence, the answer is 22 years.

Note: There is a possibility that one may commit a mistake while applying the formula for the amount. It is a very common mistake that one forgets to divide p×r×t term by 100 in the formula we used to calculate the amount and this mistake may lead us to an incorrect answer.