SOLUTION: the distance between towns m and m is 280km, a car and a lorry can travel from m to n. the average speed for the lorry 20km/hr less than that of the car. the lorry takes 1 hr 10 mi

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Question 1101380: the distance between towns m and m is 280km, a car and a lorry can travel from m to n. the average speed for the lorry 20km/hr less than that of the car. the lorry takes 1 hr 10 min more than the car to travel from m to n.
a) if the speed of the lorry is x km/hr , find x?
b) the lorry left town m at 8:15am . the car left town m later and overtook the lorry at 12:15pm. calculate the time the car left town m.
Answer by josgarithmetic(39625) About Me  (Show Source):
You can put this solution on YOUR website!
If you only look at part (a),
(Doing for speed of the car; not the lorry) 
                  SPEED        TIME        DISTANCE
CAR               r            t            280
LORRY            r-20          t+1&1/6      280

and using the given time difference,
                  SPEED        TIME        DISTANCE
CAR               r            280%2Fr          280
LORRY            r-20          280%2F%28r-20%29      280
Difference                     1%2B1%2F6


Solve for r to get the car's speed: highlight_green%28280%2F%28r-20%29-280%2Fr=1%261%2F6%29.
.
.
if no mistakes in algebra steps done on paper, then
r for the Car,
r%5E2-20r-4800=0

r=%2820%2Bsqrt%28400%2B4%2A4800%29%29%2F2

r=%2820%2B2%2Asqrt%284900%29%29%2F2

r=10%2B70

highlight%28r=80%29, for the car;
r-20=highlight%2860%29, for the lorry