Simple interest on a sum at $r\%$ per annum for $r$ years equals the principal. The value of $r$ is

Simple interest on a sum at $r\%$ per annum for $r$ years equals the principal. The value of $r$ is
  1. $5$
  2. $10$
  3. $12.5$
  4. $15$

Solution

SI $= P \Rightarrow \dfrac{P \cdot r \cdot r}{100} = P \Rightarrow r^{2} = 100 \Rightarrow r = 10$.

Asked in: IMO

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