# Primary 5 Math for September - Percentage and Average

Updated: Feb 3

**Here's what students can expect for Primary 5 Math for the month of September.**

In September, P5 students will be continuing Percentage and will start doing Average.

__In the month of August, we mentioned that we will look at word problems on percentage this month (September)__

**First**, let's look at word problems on ** GST**, the 7% GST is calculated based on the goods and services charged.

** For example**, the amount of Goods and Services that is being charged is 100% and the GST is 7%. They must know that the total being charged is 107%

Next one, word problems on ** Annual Interest**. The annual interest is calculated based on the invested/loan amount.

The original invested or loan amount is 100%. Annual interest is a certain percentage that will be given or asked to find. Total received at the end of one year will be **100% + Percentage of interest**.

The last one, word problems on __Discounts__.

The usual price or the original amount is 100%. The final selling price is 100% - Percentage of Discount.

**NOTE:** It is important to know which value is 100% and how many percent is the question asking for.

**What about the topic on Average? What are the concepts taught?**

For this topic, students need to remember the following formulas.

Generally, Average is pretty straight forward. You just have to remember the formulas and apply them

**Here is a challenging question we would like to share with you:**

For this question, we will have to take note of these three things:

At first the average is 72

3 students left, and these 3 students have an average of 85

The average score of the remaining students became 69.

Let's look at, if the students who are left, their average is 69, we just treat each of them has 69 marks. So when we draw the model, we just draw a few 69. We do not know how many, so we will use the dots to represent the rest of it.

And these 3 students are scoring 85 marks on an average. So, there will 3 of them who will score 69 + 16.

The the next part, if these 3 students did not leave the class, their average is actually 72.

So the difference between the old average and the new average is 3.

As you can in the model below highlighted in Red, we will put in the 3 marks. As for the last 3 students, we will take out 3 marks from their extra of the 16, which gives them the Green part which is 13 marks each.

These 3 sets of 13 is actually the total of the extra 3 marks. So the total of the Green part will be 13 x 3 which gives them 39. We divide by the difference of each students which is 3, we will know there are 13 students remaining. And do not forget to add back the 3 that left, so there are actually 16 students at first.

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