Lesson 5 β€” Activity 4: Estimating and Calculating Percents


Getting Ready



In the previous activity, you practised writing percents as decimals. That work will help you in this next activity.


This image shows a quick and easy way to convert a decimal to a percent.

Canada has a 5% General Sales/Service Tax (GST) on most items. Some provinces have an additional Provincial Sales Tax (PST) that is added to the cost of a purchase.


Here are some examples of products with the GST added to the cost of the item. First the 5% was changed to a decimal number to calculate: 5% to 0.05.


Item and Price
GST Added
Total Cost
Blender - $89.00
$89.00 x 0.05 = $4.45
$89.00 + $4.45 = $93.45
A chocolate bar - $0.69
$.69 x 0.05 = $0.03
$0.69 + $0.03 = $0.72
Ski jacket - $159.00
$159.00 x 0.05 = $7.95
$159.00 + $7.95 = $166.95

As mentioned previously, some provinces have an additional Provincial Sales Tax (PST) which is also added to the cost of a purchase.

British Columbia has a 7% PST, as well as the 5% GST. That's an additional 12% (0.12) to the cost of an item.


If you were in B.C., your cost for a ski jacket would be:

GST and PST Table

A simpler way to figure out the GST and the PST would be to estimate the cost of the GST and PST by adding 10% of the cost.

Remember that 10% in decimal form is 0.10.



Ski clothes will have tax added to their cost.

Looking at the ski jacket once again:


The cost is $159.00 x 10% (0.10) = 15.90 + 159.00 = $174.90. You could estimate that the ski jacket would cost a little more than this. (It actually costs $178.08)


Let's try another one:


You are looking to buy ski pants as well while in B.C. They are on sale for $97.00. Let's estimate the cost with GST and PST added.

Again, let's use 10%:

$97.00 x 0.10 = $9.70 + $97.00 = $106.70.

You could estimate that the ski pants will cost a little more than this in total.


If you need to know the exact amount, you can calculate the cost including GST and PST by multiplying by 1.12 because 100% plus 12% is the same as 1.12.

The ski pants would cost exactly $97.00 x 1.12 = $108.64. (Your estimate was very close.)




Try This:

Calculate the GST (5% or 0.05) and the total costs on these items:


  • T-shirt, which sells for $15.95


  • sunglasses, which sell for $49.98


  • wallet, which sells for $11.88

T-shirt: $15.95 x 0.05 = 0.80 + $15.95 = $16.75


sunglasses: $49.98 x 0.05 = $2.50 + $49.98 = $52.48


wallet: $11.88 x 0.05 = 0.59 + $11.88 = $12.47




Remember, you can also use the % key on your calculator to calculate a percent. Type the number x 5% to find out the amount of GST, or type the number + 5% to find out the total including GST.


Money and Decimals

Here's a review for you:

  • The decimal number 0.05 is the same as 5 pennies out of 100 and can be represented as 5%.
  • The decimal number 0.69 is the same as 69 pennies out of 100 and can be represented as 69%.
  • The decimal number 1.45 is the same as 145 pennies and can be represented as 145%.


Sometimes you may see this kind of ad when looking for music to download:


You may see this kind of offer online.Hurry!

Limited Time Offer!


This certificate earns you 20% off the cost of music of your choice if you download today!

The music Les wants costs $16.99. He is planning to use his 20% off certificate. He has exactly $13.75.

With the 20% discount, will Les have enough money to buy the music?


Let’s find out:



Step 1: Write down the original price of the item: $16.99.

Step 2: Convert how much percent off: 20% =  0.20.

Step 3: Multiply the original price by the decimal number and place the decimal in the appropriate position: $16.99 x 0.20 = $3.398.

Step 4: Round this number to the nearest hundredth: $3.40.

Step 5: Write down the original number and subtract: $16.99 – $3.40 = $13.59. This is the new sales price with the 20% discount.
Les has enough to get the music he wants.

Try This:


This problem involves the sale of a scooter.

Jackie sold her scooter for 20% less than what she paid for it. She paid $455.00.
How much did Jackie get when she sold her scooter?


  • Find 20% of $455.00.
  • Subtract this amount from $455.00.
  • Find out how much she got for her scooter.



$455.00 x 0.20 = 91.00
$455.00 – $91.00 = $364.00
Jackie received $364.00 for her scooter.


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