Quantitative Aptitude :: Time and Distance - Discussion

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Explanation:
Let the distance travelled at the rate of 12 kmph be x km
Time taken to travel this distance = x/12 .......(1)
Then the distance travelled at the rate of 10 km/hr will be (75 - x)
Time taken to travel this distance = (75 - x)/10 .......(2)
Total time taken to cover the total distance = 7 hrs
=>(x/12) + (75 - x)/10 = 7 [sum of (1) and (2)]
=> (10x + 900 - 12x) = 840
=> -2x = -60
=> x = 30.
Therefore, the distance travelled at the rate of 12 km/hr = 30km.