中一數學問題

Mr.Chan took 2 hours to drive from A to B. The average speed of the whole journey was 60 km/h. If his driving speed for the first 1/3 of the journey was 45 km/h, find

(a) the driving time for the first 1/3 of the journey

Ans: 2hoursx1/3=120mins x1/3=40 mins

(b) the driving speed for the remaining journey

for the Frist 1/3 jorney ,, he has traveled 40minsx45=0.6666hrs x45=30 km...

and the remaining journey is 2x60-30=90 km.....

and the time is 2x(2/3)=1.333333 hrs

so the speed is = distance / time = 90/1.3333333=67.5 km/hrs

The total distance of the journey is 60*2 = 120km

(a)
1/3 of the journey is 40km.
the driving time for the first 1/3 of the journey is 40/45.
8/9 hour.

(b)
The remaining distance is 80km.
the remaining time is 2-8/9 = 10/9.

driving speed is 80/(10/9) = 72km/h.

(a) the driving time for the first 1/3 of the journey.