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# Rate, Ratio and Proportion Examples

## Example

• John can finish a large chocolate bar in three hours
• Michael eats the same chocolate bar in two hours
• Nathaniel can eat the chocolate bar in 5 hours

Assuming that their rate of eating doesn’t change and that they start eating at the same time, how long does it take all three of them to finish one large chocolate bar?

Observation: If Michael can finish the chocolate bar in two hours, then all three of them should finish the chocolate bar in less than two hours.

We find the least common multiple of and . This value is .

So, given 30 hours, John eats 10 chocolate bars. Given 30 hours, Michael eats 15 chocolate bars. Also given 30 hours, Nathaniel eats 6 chocolate bars.

So, given 30 hours, John, Michael and Nathaniel eat a total of chocolate bars.

To eat 31 chocolate bars takes 30 hours and to eat 1 goat takes mins, secs, correct to the nearest second.

## Example

A sum of money is divided in the ratio . Given that the smallest share is £45. Find the largest share of money.

Let x = sum of money, so we are given:

Solving for x gives

.

Largest share

## Example

What is the ratio of £50 to £125?

A)

B)

C)

D)

The ratio of £50 to £125 is:

(fraction)

(In ratio form)

## Example

Given that y varies directly as and that when , . Find the value of y when x is 3.

, where k is a constant

Substituting x=2, y=125 gives:

Hence, the connecting equation between x and y is:

Now when ,

## Example

• Anne and Betty can complete a job in four hours
• Anne and Clara can complete the job in six hours
• Betty and Clara can complete the job in nine hours

How long does it take if Anne, Betty and Clara work together? Assuming they work at the same rate.

A) 9 hours, 30 minutes

B) 6 hours, 20 minutes

C) 6 hours, 33 minutes

D) 3 hours, 47 minutes

Anne and Betty can complete the job in four hours. The percentage (%) of the job Anne does is 4 hours + the percentage of the job Betty does in 4 hours, which equals 100% or 1.

4(% job Anne does in 1 hour) + 4(% job Betty does in 1 hour) = 1

(i)

Likewise, Anne and Clara can complete the job in six hours. So, the percentage Anne does in 6 hours + the percentage Clara does in 6 hours = 100% or 1.

(ii)

For Betty and Clara, we have (iiii)

So, we need to find , for some value k.

The lowest common multiple (LCM) of 4, 6 and 9 is 36.

So, multiply (i), (ii) and (iii) BY 9, 6 and 4, to obtain:

Adding the last three equations, we have:

The time taken for Anne, Betty and Clara working together:

hours

= 3 hours, 47 minutes

So, the answer is D = 3 hours, 47 minutes.

## Example

If and , then equals…

A) 4:5

B) 28:75

C) 75:28

D) 14:75

Rewriting these ratios as fractions, we get:

Now

As a ratio, this is , so the answer is B.

## Example

Suppose that there are 3 bells:

• Bell A
• Bell B
• Bell C

Bell A rings every three minutes, bell B rings every five minutes and bell C rings every six minutes. They last rang together at 9:00 AM. When will they ring together again?

You can draw a table showing the successive ringing times for each bell, which you can see below.

Bell A Bell B Bell C
9:00 9:00 9:00
9:03 9:05 9:06
9:06 9:10 9:12
9:09 9:15 9:18
9:12 9:20 9:24
9:15 9:25 9:30
9:18 9:30 9:36
9:21 9:35
9:24
9:27
9:30
9:33

Then we look for the time that is common in all three columns. The least number we can see in all three columns is 9:30 AM. So, the three bells will ring together at 9:30AM.

However, we usually solve this type of problem using least common multiple (LCM).

We find the LCM of 3, 5 and 6 minutes.

LCM

= 30 minutes

So, the three bells will ring together 30 minutes after 9:00 AM.

9:00 AM + 30 minutes = 9:30 AM

## Example

If five people can dig a ditch in 16 days, how long will it take eight people to dig the ditch?

If five people take 16 days, one person will take days, which is 80 days.

So, eight people will take days to dig the ditch

## Example

A book has 540 pages and is 2.7 cm thick, ignoring the covers. How many pages are there in a book that is 3.7 cm thick?

2.7 cm → 540 pages

1 cm  pages

3.2 cm pages.