Maths-probability

2010-05-08 8:15 pm

請教以下問題，謝謝！

(3) Ann asks her brother and sister to buy her snacks. Her brother has three $10 coins and one $2 coin. Her sister has two $10 coins and two $5 coins. If they each pick a coin out and give them to Ann, find the probability that
(a) Ann receives $20,
(b) Ann receives less than $15.

(4) At the entranceof Paradiso Hotel, a shuttle bus comes in every 15 minutes and it waits 3 minutes at the bus stop. If a guest of the hotel does not know the timetable of the shuttle bus service, find the probability that he can get on the bus without waiting.

(5) Every day Betty goes to Cafe One for tea from 3:30 p.m. to 3:45 p.m. Today Charles will have a tea at Cafe One for 10 minutes. If Charleswill enter Cafe One at any time from3:00 p.m. to 4:00 p.m., what is the probability that he will meet Betty there?

回答 (1)

用戶2053

2010-05-08 10:34 pm

✔ 最佳答案

3a)
P(Ann receives $20)
= P(brother pick $10) * P(sister pick $10)
= 3/4 * 2/4
= 3/8
b) P(Ann receives less than $15)
= 1 - P(Ann receives $20) - P(Ann receives $15)
= 1 - 3/8 - P(brother pick $10) * P(sister pick $5)
= 1 - 3/8 - 3/4 * 1/2
= 1/4
Method 2 :
= 1 - P(brother pick $10) * P(sister pick any coin)
= 1 - 3/4 * 1
= 1/4

4)P(he can get on the bus without waiting)
= 3/15 (Since waits 3 minutes in every 15 minutes)
= 1/5

5)
P(he will meet Betty there)
= P(he enter Cafe from 3:20p.m. to 3:45p.m.)
= (45 - 20) / 60
= 5/12

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