John covers a part of the journey at $20 k m / h r$ and the remaining at $70 k m / h r$ , taking a total of $8$ hours to cover the distance of $400 k m$ . How many hours he has been driving at $20 k m / h r$ ?

2 hours

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3 hours 20 minutes

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4 hours 40 minutes

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3 hours 12 minutes

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Solution

The correct option is D
3 hours 12 minutes

Let the total time he travelled with a speed of $20 k m / h r$ is $x h r$ .

So the time he travelled for at $70 k m / h r$ = $8 - x$

So, $20 x + 70 (8 - x) = 400$

The total distance traveled = $20 x + 560 - 70 x = 400$

$50 x = 160$

$x = 3.2 h r s$

i.e. $3 h o u r s 12 m i n u t e s$

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