Question 920768
let v- constant speed of car in km/hr
    s- distance between the two cities(P to Q) in kilometers
    t- time to travel from P to Q in hours
    working formula: s=vt
    cond. 1: s=vt >(Eq 1)
    cond. 2: v is increase by 10 km/hr, the value of (v) in (Eq 1) becomes
                 (v+10), and (t) in will be (t-1)  
                 s=(v+10)(t-1) > (Eq 2)
    cond. 3:v is increased further by 10km/hr, time is cut by 45 minutes
                  s=(v+20)(t-7/4) >(Eq 3)... (7/4= 1 hr and 45 minutes)
    since distance from P & Q is constant, s will be same in all conditions
              equate (Eq 1), (Eq 2) and (Eq 3) 
              v=60km/hr; t=7 hours
              s=420 km-the distance you are looking for...