Please explain me how to get the answer to this question...

Dear student,
Let the distance between his house ans station be D km.
When Sahil walks at 5 km/hr, 
He travels 5 km in 60 minutes
He travels 1 km in 60/5=12 minutes
He travels D km in 12D minutes.

​​​​​​When Sahil walks at 6 km/hr, 
He travels 6 km in 60 minutes
He travels 1 km in 60/6=10 minutes
He travels D km in 10D minutes.

In the first case, he misses the train by 7 minutes and in the second case, he reaches station 5 minutes before departure.
so, difference of time in these two cases is (7+5)=12 minutes.

According to the condition,
$12D-10D=12$
or, $2D=12$
or, $D=6$
Hence, the required distance  is 6 km.

Regards.

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