SWC#2 Ratio tables

• Wiskunde
• Middelbare school
• havo, vwo
• Leerjaar 1

scientific calculations week #2: RATIO TABLEs

GOAL of this lesson:

HOW TO CALCULATE IN A RATIO TABLE

What is a RATIO?

It's a 'verhouding' in Dutch.

Example 1: 

20 cookies fit into 1 package

the RATIO = 20:1

Example 2

you need 200 grams of flour to bake 2 muffins,

so the RATIO is 200 : 2, or better:          100:1

cookies	20	40	80
pack	1	2	4

Why is 100:1 better than 200:2 ?

Explanation:

Both expressions mean the same.

However a RATIO is always written with:

THE SMALLEST POSSIBLE WHOLE NUMBERS ! 

How the calculate in a ratio table?

example 1 

Look above! It's about the euros earned by working  a number of hours.

Think about how to get the numbers on the dots.

euros	5	15	90
hour	2	...	...

Solution:

Multiplying HORIZONTALLY is simplest here:

5 x 3 = 15, so for 15 hours worked you get 2 x 3 = 6 euros

5 x 18 = 90, so for 90 hours worked you get 2 x 18 = 36 euros

euros	5	15	90
hours	2	6	36

Do you prefer to calculate vertically or horizontally, now?       ANSWERS:

example 4:

Let us try to find the missing number!

In the next slide I will show you a handy METHOD.

grams	500	240
euros	750	...

What's up, now?

Well, if we first calculate the price in euros for just one gram,

the rest is quite easy!

Think about the missing numbers.

grams	500	1	240
euros	750	...	...

Explanation:

Now that we know that 1 gram costs 1.5 cents,

we do:

240 x 1.5 = 240 + 120 = 360 cents.

grams	500	1	240
euros	750	1.5	360

The book calls this Method:

'adding a number as an INTERMEDIATE STEP (=tussenstap)'

This intermediate step is mostly the number 1.

However it could also be 10,   50    or any handy number!

It depends on the concrete situation.

grams	500	1	240
euros	750	1.5	360

example 5:

TASK:

Find the easiest way to get the missing numbers.

grams	200	20	220
cents	600	...	...

example 6:

Again find the missing numbers.

grams	40	2	38
cents	160	...	...

example 6:

Solution:

This time SUBTRACTING does the trick!

This is because 40 - 2 = 38

First we calculate that 2 grams cost 8 cents.

Then we do       160 - 8 = 152 cents.

grams	40	2	38
cents	160	8	152

Get to work

Now do task 1-3

Always make and work out RATIO TABLES! Plus write down a seperate answer