速率問題急

🎓 學業台小學及中學教育

[匿名]

2011-08-09 5:11 am

At noon, Peter drives from town A to town B at a speed of 70 km/h . At the same time , Mary,and Ann drive from town B to town A at speeds of 50 km/h and 40 km/h respectively. Mary arrives at town A 15 minutes earlier than Ann. Suppose that the distance between the two town is d km.

a)Express the time taken by Mary and the time taken by Ann respectively to travel from town B to town A in terms of d .

b)hence, find the value of d.

c)If the time taken for peter and mary tomeet each other is t hours, express the distance travelled by peter and mary respectively before they meet each other in terms of t.

d)At the time will peter meet mray?

請用中文解釋

更新1:

sorry d)At the time will peter meet mary?

回答 (2)

Godfrey

2011-08-09 4:17 pm

✔ 最佳答案

a.
速率 = 距離/時間, 時間 = 距離/速率
城鎮A與城鎮B距離 = d km, Mary速率 = 50 km/h, Ann速率 = 40 km/h

Mary從城鎮B到達城鎮A需要時間 (h, 小時) = d/50
Ann從城鎮B到達城鎮A需要時間 (h,小時) = d/40

b.
15 min. = 15/60 h = 1/4 h (小時) ~ Mary早過 Ann 15分鐘到達
Mary從城鎮B到城鎮A需時間 - Ann從城鎮B到城鎮A需時間 = Mary快過Ann時間到達
d/40 - d/50 = 1/4
50d/2000 – 40d/2000 = 1/4
(50d-40d)/2000=1/4
10d /2000 = 1/4
d = 50

d 是 50 km

c.
Peter速率 = 70 km/h, Mary速率 = 50 km/h, t 是所需時間 (小時) Peter相遇 Mary
換句話, 經過 t小時後, 他們合共行了50 km

經過 t小時後, Peter 行了路程 = 70 t
經過 t小時後, Mary 行了路程 = 50 t

d.
經過 t小時後,
Peter行了路程 + Mary行了路程 = 50 km
70t + 50t = 50
120 t = 50
t = 50/120 = 5/12小時 = 25 min (分鐘) ~ 5/12 (60) = 25 min.

經過25 分鐘, Peter 會相遇 Mary.

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[匿名]

2011-08-09 7:23 am

a.
d/40-d/50=1/4
呢度用距離除速率得出時間...而1/4係因為佢單位用h(小時)...i.e.15mins=1/4小時

b.
d/40-d/50=1/4
(50d-40d)/2000=1/4
10d=500
d=50

c.
70t+50t=50
呢度係速率乘時間=距離

d.
Peter will meet Mary
但你係咪想問幾耐Peter will meet Mary?
我打埋啦...姐係要搵個t
70t+50t=50
120t=50
t=50/120
t=5/12小時

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