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A man covers a certain distance on car. Had he driven 21 kmlr1 faster, he would have taken 7 hours less. If he had driven the car 13 kmlr1 slower, he would have taken 13 hours more. What is the distan

Answer»

510 km



Let the DISTANCE be x km and speed y kmh-1
or, x/y = x/(y + 21) + 7
or, x/y - x/(y + 21) = 7
or, x (1/y - 1/y + 21) = 7 ...... (I)
x/y = x/(y - 12) = - 13 ...........(II)
Dividing (I) HY (II),
 1/y - 1/(y + 21)
= -7/13 (1/y - 1/y - 13)
or, y + 21 - y/y(y + 21)
= -7/13 (y - 13 - y/y(y - 13))
or, 21/y(y + 21) = 7/y(y - 13)
∴ 3/y + 21 = 1/y - 13
or, 3y - 39 = y + 21
or, 2y = 60


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