Speed increases from $40$ to $60$ km/h, completing the trip $1$ hour earlier. Distance is

Speed increases from $40$ to $60$ km/h, completing the trip $1$ hour earlier. Distance is
  1. $120$ km
  2. $60$ km
  3. $100$ km
  4. $80$ km

Solution

$\dfrac{d}{40} - \dfrac{d}{60} = 1 \Rightarrow \dfrac{d}{120} = 1 \Rightarrow d = 120$.

Asked in: IMO

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