20 Days to Go(-7 Days): Probability 重溫

Q1-

There are 4 different colors of balls in a bag and 5 balls of each color. If 4 balls are drawn randomly from the bag without replacement, find the probability that exactly 3 different colors are obtained.

想睇2019年DSE Math LQ同MC既參考答案CLICK呢度啦！

-同學仔普遍不能準確處理呢類問題，相信最主要係有兩個原因︰未能準確分cases及數不清楚每個cases的情況。

呢類問題，通常可以次數量分cases。

-首先，按數量來講，題目要求三種顏色，明顯只有AABC呢個組合。即係兩粒A顏色、一粒B顏色同埋一粒C顏色。

由於A顏色有兩粒而B及C顏色只有一粒，所以要分開計算。從4種顏色中揀一種顏色是A，再從剩低的3種顏色中揀兩種是B及C，即是solution 中的 4C1及3C2。

然後，在A顏色中，要選兩粒，所以有。在B及C顏色中，各要選兩粒，所以需要二次。

-同學仔需要有個習慣，就係數數目時，將揀野嘅次序諗清楚，分好層次。

俾多題同學仔試下喇。

In a company, there are 8 departments and 3 employees are in each departments. Now, 5 employees are chosen from these 24 employees. How many different combinations are there if the chosen employees are come from exactly 3 different departments?

(6048)