If $y$ varies directly as $x$ and $y = 30$ when $x = 5$, find $y$ when $x = 9$.

If $y$ varies directly as $x$ and $y = 30$ when $x = 5$, find $y$ when $x = 9$.
  1. $45$
  2. $50$
  3. $54$
  4. $60$

Solution

$k = \dfrac{y}{x} = \dfrac{30}{5} = 6$. Then $y = 6 \cdot 9 = 54$.

Asked in: IMO

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