A car travels from P to Q at a constant speed. If its speed were increased by 10 kmph, it would have taken one hour lesser to cover the distance. It would have taken further 45 minutes lesser if the speed was further increased by 10 kmph. What is the distance between the two cities?
a) 420 km b) 540 km
c) 600 km d) 650 km

• Let d be the distance... s nd t be the original speed nd time ...
  Then d=st
  If speed increased by 10,
  d = (s+10)*(t-1)
  d = st-s+10t-10
  d = d-s+10t-10
  10t-s = 10 --->(1)
  
  If speed increased by further 10,
  d = (s+20)*(t-1*3/4)
  d = st- 7s/4 + 20t -35
  d = d - 7s/4 + 20t - 35
  20t - 7s/4 = 35 --->(2)
  
  Solving (1) nd (2), we get
  t = 7 hrs
  s = 60 kmph
  d = 60*7 = 420 km
  
  Ans : (a) 420 km
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