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8/14/2019 BUKSIS-5.4 BARU

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What are you goingto learn? To c om prehend

inverse proportion

Key Term: inverse proportion

Lia is having a birthday party. Lia has 12 donuts that will be

given to each friend equally.

If Lia invites two friends, how many donuts will each of them

receive?If Lia invites three friends, how many donuts will be

received by each person?

Fill in the blanks:

Invited friends Donuts for each

2 .......3 .......

4 .......

...... 2

...... 1

If there are morefriends to invite, what happens

to the donuts for each?

If the ratio of the number of friends invited is2 : 3, what is the ratio of the number of donuts

for each?

If the ratio of the number of friends invited is

3 : 4, what is the ratio of the number of donuts

for each?

If the ratio of the number of donuts for each is

21 , what is the ratio of friends?

What can you say about the ratio of the number

of friends who are invited and the ratio of the

number of donuts for each?

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The above problem is called an inverse proportion.

Mathematics for Junior High School Year 7 /201


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Problem 1

For mathematics lesson,

the class will be divided

into several groups. Thenumber of students for

each group is the same.

The number of students

for 3 groups is 9. If there

are 13 groups in the

class, what is the total

number of students in

the class?

WorkTogether

Suppose the floor of a bedroom has the size of 3 m x 2.4 m.

The floor will be covered with tiles. If the size of each tile is

30 cm x 30 cm, how many tiles are needed to cover the

floor?If the size of each tile is 20 cm x 20 cm, how many tiles are

needed to cover the floor? If you have a problem, you can use

pieces of papers to represent tiles.

The price of one 20 cm x 20 cm tile is Rp 1,700.00 .

The price of one 30 cm x 30 cm tile is Rp 3,600.00.

The labor fee is Rp 15,000.00 per m2. Complete this table:

PriceSizeof tile

Number

of tiles

Price of

tiles(Rp)

Fee

(Rp)

Total

Cost(Rp)

20 cm 20 cm

30 cm 30 cm

/ Students Book - Proportion202


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Based on the above table, which size of tile must be chosen in order

to get the lower cost?

Twenty five workers can construct a house in 72 days. If there are

only 18 workers, how many days do they need to construct a house?Problem 3

Questions to Solve Problem

If there are fewer workers to construct a house, what happens to the

number of days needed to finish the construction?

If the number of days that 18 workers need to build a house is x, can

you write a proportion for that problem?



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1. Wawan must bring water in a 2-litre pail for 24 times to fill in thebath-tub. If Wawan wants to bring water only 16 times, how

many pails of water should Wawan bring?

Suppose the capacity of the pail that he must bring is x liter.

Write the proportion for Wawan's problem. Then find x.

2. Toto breaks his piggy bank. If the price of a book is Rp 1,300.00,

then Toto can buy 12 books with all his money. The price of apen is Rp 1,950.00. If Toto wants to spend all money for pens,

how many pens can Toto get?

Suppose the number of pens that he can buy is p. Write the

proportion for Toto's problem. Then, findp.

3. Mr. Imam's farm can be cultivated by 15 workers in 6 days.

Suppose there are only 9 workers. How many days can Pak

Imam's farm be cultivated completely?

4. A rectangle has the length of 24 cm and the width of 18 cm.

Suppose the length is 20 cm. What is the width in order to

get the same ratio?

5. Edi buys 5 fried bananas with all his money. He only gets 3

glasses of lemonade with the same amount of money.

Suppose one fried banana costs Rp. 300.00. How much does

a glass of lemonade cost?

6. A drawer can accommodate 36 books, each of which is 8

millimetres thick. How many 12-millimetres books can be

put into that drawer?



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7. A ship can be constructed by 45 workers in 24 days. Suppose

there is an order to make a ship in 18 days. How many

workers are needed for it?

8. A truck can carry 364 packages of rice. A package of rice weighs

50 kg.

Suppose a package of sugar weighs 40 kg. How many packages

of sugar can the truck carry?

9. A label in a bottle of medicine

is as shown on the figure on the

right side.

Suppose a 7 year old child candrink all in 8 days. How many

days does a 15 years old teenager

need to drink all?

Direction

6 - 12 years : 3 1 teaspoon

12 years and up : 4 1 teaspoon

10.Critical ThinkingDetermine the proportion of length and

width of a rectangle if the length is doubled and the width

remains the same.



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