2Tvwo § 2.1 ratio tables '19-'20

• Wiskunde
• Middelbare school
• havo, vwo
• Leerjaar 2

§2.1 Ratio table

In first form you already worked with ratio tables.

But this looks like ages ago...

That's why this explanation follows now.

Calculating with percentages in a ratio table

When working with percentages, a ratio table is an easy way to show your calculations.

This works for questions where you are given the amounts and have to calculate the percentage, but also for questions where you are given the percentages and have to calculate the amount.

Only round off your final answer!

Calculating with percentages in a ratio table

Example: amount --> percentage

In class 2R, 21 pupils cycle to school, 7 take the bus and 2 walk to school. What percentage of pupils takes the bus?

Calculating with percentages in a ratio table

Example: amount --> percentage

In class 2R, 21 pupils cycle to school, 7 take the bus and 2 walk to school. What percentage of pupils takes the bus?

• Make a ratio table with the total number of pupils (100%), an empty column and the number of pupils asked (7)
  

Calculating with percentages in a ratio table

Example: amount --> percentage

In class 2R, 21 pupils cycle to school, 7 take the bus and 2 walk to school. What percentage of pupils takes the bus?

• Make a ratio table with the total number of pupils (100%), an empty column and the number of pupils asked (7)
• The known numbers are in the top row, so                                                                 calculate the percentage for 1 pupil and use                                                            this to calculate the percentage for 7 pupils.
  

Calculating with percentages in a ratio table

Example: amount --> percentage

In class 2R, 21 pupils cycle to school, 7 take the bus and 2 walk to school. What percentage of pupils takes the bus?

• Make a ratio table with the total number of pupils (100%), an empty column and the number of pupils asked (7)
• The known numbers are in the top row, so                                                                 calculate the percentage for 1 pupil and use                                                            this to calculate the percentage for 7 pupils.
  

Calculating with percentages in a ratio table

Example: amount --> percentage

In class 2R, 21 pupils cycle to school, 7 take the bus and 2 walk to school. What percentage of pupils takes the bus?

Answer: 23% take the bus.

Important: Do NOT round of the

percentage for 1 pupil on your

calculator. Short calculation:

Calculating with percentages in a ratio table

Example: percentage --> amount

In class 2R, there are 30 pupils. 47% of the pupils scored above a 9 on the test of chapter 2. How many pupils were this?

Calculating with percentages in a ratio table

Example: percentage --> amount

In class 2R, there are 30 pupils. 47% of the pupils scored above a 9 on the test of chapter 2. How many pupils were this?

• Make a ratio table with the total number of pupils (100%), an empty column and a column with the percentage asked (47%)

Calculating with percentages in a ratio table

Example: percentage --> amount

In class 2R, there are 30 pupils. 47% of the pupils scored above a 9 on the test of chapter 2. How many pupils were this?

• Make a ratio table with the total number of pupils (100%), an empty column and a column with the percentage asked (47%)
• The known numbers are in the bottom row, so                                                      calculate how many pupils go with 1% and use                                                      this to calculate how many pupils go with 47%
  

Calculating with percentages in a ratio table

Example: percentage --> amount

In class 2R, there are 30 pupils. 47% of the pupils scored above a 9 on the test of chapter 2. How many pupils were this?

• Make a ratio table with the total number of pupils (100%), an empty column and a column with the percentage asked (47%)
• The known numbers are in the bottom row, so                                                      calculate how many pupils go with 1% and use                                                      this to calculate how many pupils go with 47%
  

Calculating with percentages in a ratio table

Example: percentage --> amount

In class 2R, there are 30 pupils. 47% of the pupils scored above a 9 on the test of chapter 2. How many pupils were this?

Answer: 14 pupils scored above a 9.

IMPORTANT: Round off sensibly.

You cannot have 0.1 pupils.

Homework:

• Bring a pair of compasses (and your usual stuff) !
• Make the exercises from your chosen path from §2.1

Support: 1, S2, 3, 4, 5, S6, 6

Standard: 1, 2, 3, 4, 5, 6, 7

Challenge: 1, 2, 3, 5, 6, C1, C2

• Correct the exercises with the solutionsbook
• Send in pics of your work in GC.