If $x$ and $y$ are in inverse proportion and $x = 4$ when $y = 15$, the constant $k = xy$ equals:

If $x$ and $y$ are in inverse proportion and $x = 4$ when $y = 15$, the constant $k = xy$ equals:
  1. $11$
  2. $19$
  3. $45$
  4. $60$

Solution

$k = xy = 4 \times 15 = 60$.

Asked in: IMO

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