A sum amounts to Rs. $11{,}000$ in $4$ years and to Rs. $12{,}500$ in $6$ years at simple interest. The…

A sum amounts to Rs. $11{,}000$ in $4$ years and to Rs. $12{,}500$ in $6$ years at simple interest. The principal is
  1. Rs. $7{,}500$
  2. Rs. $8{,}000$
  3. Rs. $8{,}500$
  4. Rs. $9{,}000$

Solution

SI for $2$ years (from yr 4 to yr 6) $= 12500 - 11000 = 1500$, so SI/year $= 750$. SI for $4$ years $= 3000$. So $P = 11000 - 3000 = 8000$.

Asked in: IMO

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