If $a$ varies inversely as $b$ and $a = 9$ when $b = 8$, the value of $b$ when $a = 18$ is:

If $a$ varies inversely as $b$ and $a = 9$ when $b = 8$, the value of $b$ when $a = 18$ is:
  1. $2$
  2. $3$
  3. $4$
  4. $16$

Solution

$ab = 9 \times 8 = 72$. When $a = 18$: $b = \dfrac{72}{18} = 4$.

Asked in: IMO

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