Question : Flight A usually takes 1 hour more than Flight B to travel a distance of 7200 km. Due to engine trouble, the speed of Flight B falls by a factor of $\frac{1}{6}$th, so it takes 36 minutes more than Flight A to complete the same journey. What is the speed of Flight A (in km/h)?
Option 1: 800
Option 2: 900
Option 3: 750
Option 4: 720
Correct Answer: 800
Solution :
Let the speed of flight A be $x$ km/h and the speed of flight B be $y$ km/h.
$\frac{7200}{x} = \frac{7200}{y}$+1-------------------------------(i)
After engine trouble, speed of flight B = $x–\frac{x}{6} = \frac{5x}{6}$ and it takes 36 minutes more than flight A.
$\frac{7200}{x} = \frac{7200×6}{5y}–\frac{36}{60}$-------------------------(ii)
Equating both equations,
$\frac{7200}{y}$+1 = $\frac{7200×6}{5y}–\frac{36}{60}$
⇒ $\frac{7200}{5y}$ = 1 + 0.6
⇒ $\frac{1440}{y}$ = 1.6
⇒ $y$ = 900 km/h
From equation(i), $x = 800$ km/h
The speed of flight A is 800 km/h.
Hence, the correct answer is 800 km/h.
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