Maths F.3 urgent!!pls

1.
Let P be the principal.
Let T be the required time to double the money.

Recall:
　Interest = Principal × Rate × Time
　Amount = Principal + Interest

2P = P + P × 5% × T
2P = P(1 + 5% × T)
2 = 1 + 5% × T
1 = 5% × T
1 = 0.05 × T
T = 1 ÷ 0.05
T = 20

Therefore, it takes 20 years to double the original sum of money.

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2.
Discrete data (散離數據) are those data which can only take certain values. One example is the count data such as the number of students in a school. Integers are discrete type. If you are given a range, for example, from 1 to 4, you can only take finite values: 1, 2, 3, 4.

Continuous data (連續數據) are those data which can take any value in a given range. One example is the height of a student in your class. The height can take any value in between a valid range, for example, from 80 cm to 85 cm, you can take infinite many different height values, such as 81, 82.6, 82.99999, 88.123, 89, ... etc.

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3.
Ratio (比例) is the comparision of two quantities of the same type by division, there is NO unit for ratio (because they can cancel each other).
For example, the ratio of numbers of boys and girls is 3 : 8.
For example, the ratio between two length is (20 cm) : (1 m) = (20 cm) : (100 cm) = 20 : 100 = 1 : 5.

Rate (率) is the comparision of two quantities of different types by division, there must be unit for rate.
For example, the speed is the rate of distance to time.
Speed = Distance / Time = (200 m)/(5 s) = 40 m/s


2015-03-22 17:44:51 補充：
補充一下：
Discrete data 是 離散數據，你可以看看：

http://zh.wikipedia.org/zh-hk/%E9%9B%A2%E6%95%A3