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Mathematics in Working World 2

Let’s be realistic, we work to earn a living. Thus, the $$ (salary) becomes the first priority. There is a difference between gross salary and take-home salary.

The amount that your employer tells you is your gross salary. The gross salary deducts CPF and taxes (if any), then adds any overtime (OT) pay and any allowances, and the final amount is the take-home salary.

Take a simple example, the gross salary is $2000. With the assumption that you are 35 years old and below, you need to contribute 20% of your salary to CPF. Now you need your knowledge of percentage to calculate how much you need to contribute to CPF.

2000 x 20% = 400

With another assumption that there is no OT pay and other allowances, your take-home salary is

2000 – 400 = 1600

The good news is your employer contributes another 16% of your salary to CPF.

2000 x 16% = 320

400 + 320 = 720

Thus, your CPF account will have the total of $720 for the month.

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Now that you know your take-home salary, let’s go shopping. You want to buy a pair of shoes that costs $80. With GST of 7%, the cost of the shoes is

80 x 7% = 5.60

80 + 5.60 = 85.60

You need to pay $85.60 for the pair of shoes with GST included.

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

All these calculations are the simplified versions that you will be using when you start working. The next time you are complaining about learning Mathematics, think about how Mathematics will help you in future and you won’t complain. Happy learning!

Why Do We Learn Percentage?

This is the second post in the “Why Do We Learn …?” series. The main purpose of this series is to show the real life application of different Mathematics concepts. In this post, we continue with the topic of buying a commodity.

Below is table showing the price of the commodity with different weights:


Sell Out Price (S$)

Buy Back Price (S$)

Difference between SOP and BBP (S$)

Difference in Percentage (%)

1 g 56.50 56.10 0.40 0.71
1 kg 56300 56100 200 0.36
1 ounce 1820 1750 70 3.85
10 ounce 17900 17600 300 1.68

Concept 1: Money

The sell out price is the money that you pay to the bank when you buy the commodity.

The buy back price is the money that the bank pays you when you sell the commodity back to the bank.

From the table above, we can see that there is difference between the sell out price and buy back price and the buy back price is always lower than the sell out price.

Concept 2: Percentage

If we invest in a commodity, we would like to earn money, that’s common sense.

With the assumption that the price is the same as the above table when you buy and sell the commodity, which weight would you choose to make maximum profit?

The difference in price does not make a good indicator because the weight difference is big and thus the difference in price is big too. Thus, we should choose the difference in percentage as a better indicator.

We choose the smallest difference in percentage to make maximum profit. To calculate the difference in percentage for 1 g:

The price difference = 56.50 – 56.10

                                                                                                      = 0.40

The percentage = (0.40/56.50) x 100%

                                                                                             = 0.71%

From the table, we know that the smallest difference in percentage is for 1 kg, 0.36% and the biggest difference in percentage is for 1 ounce, 3.85%.

If the price of the commodity has increased by 2%, investment in 1 kg has positive return while investment in 1 ounce has negative return.

In a nutshell, we can decide which weight will give you a good return by using the difference in percentage, i.e. 1 kg gives the best return among the four weights while 1 ounce gives the least return.

Disclaimer: This post is not encouraging children and / or adults to invest blindly. This is only a simple example where you can use the knowledge learnt in real life. Investment in real life is more complicated and involves more risks.

Primary Mathematics Notes

Some parents have been complaining about Primary Mathematics. They say nowadays Primary Mathematics is not “pure” Mathematics anymore. They say that the Mathematics questions are more like playing with word games, if you do not understand the questions, you cannot solve the questions. Nonetheless, we can still solve the “word games” and score high in Mathematics! In addition, the terms are universal, you can keep this note for Secondary Mathematics too.

Caution: Though the wordings used in this post are commonly found in the Mathematics questions, each question is unique and may vary. The most important thing is to read and understand the questions. Treat each question on a case-by-case basis. If you face any problem on Mathematics questions, please feel free to contact me at

Below are some of the common words used in Primary Mathematics questions:

1. as … as = same

Jenny is as tall as Kenny. That means both Jenny and Kenny have the same height.

Variation version:

(a) Jenny has twice as many candies as Kenny. That means if Jenny has 6 candies, Kenny has only 3 candies. In algebra, Jenny’s candies = 2 x Kenny’s candies.

(b) When x is doubled, find y. That means when x = 2x.

2. -er than, the difference of => use subtraction (-)

Kenny has $50 more than Jenny. That means the difference of the amount of money between Kenny and Jenny is $50, most questions can be solved using subtraction.

Kenny is taller than Jenny by 2 cm. That means the difference of the height between Kenny and Jenny is 2 cm and Kenny is taller.

3. altogether, the sum of => use addition (+)

How many flowers are there altogether? The question is asking you to add all the flowers mentioned in the question.

What is the sum of money? The question is asking you to add all the money value mentioned in the question.

4. Mathematics language

(a) Subtract 5 from 9 => 9 – 5

(b) A bag cost $5.00, how much does it cost if Jenny buys 3 bags? => $5 x 3

Below are the concepts that must be understood by Primary students so that they can tackle more difficult questions:

1. Percentage

2. Ratio

3. Average

4. Bar modelling

5. Algebra (This concept will be taught in more details in Secondary school, thus the understanding of the concept is of utmost importance)

In a nutshell, understanding the concepts is the most important thing to learn in Mathematics. Once you have understood, solving more questions will reinforce the understanding and A* is on the way!