definition:


Inversely Proportional means: 

when one value increases at the same rate that the other decreases  ↑  ↓

or when one value decreases at the same rate that the other increases   ↓   ↑

 

example:


speed and drive time:

As speed goes down, drive time goes up.

As speed goes up, drive time goes down.

 

calculate:


A car is going at at speed 54 mph. It takes 20 minutes to the city.

How long does it take the same car if it is going at a speed of 60 mph?

 

1. step: proportion

* 54 mph  -  20 minutes

: 60 mph   -  x minutes

 

2. step: which proportion?

 ↑more speed  ↓less minutes  = inverse proportion

 

3. step: formula

x = 20 * 54
         60

x = 18 minutes   

 

4. step: answer

It takes the car 18 minutes to the city. 

 

problem 1   answer


8 workmen need 15 days to build a new house. 

How many workmen are needed to build the house in 10 days? 

 

problem 2  answer


6 caterpillars need 20 days to cut a canal.

How many caterpillars are needed to cut the canal within 15 days? 

 

problem 3  answer


Tom fills orange juice into forty-five 0,5-l  bottles. 

If he uses 0,75-l bottles, how man bottles can he fill with the same amount of orange juice? 

 

problem 4 answer


A car is going at at speed of 60 mph. It takes 12 minutes to the city.

How long does it take the same car if it is going at a speed of 80 mph?

 

problem 5 answer


The supply of hay of a farmer lasts for 150 days to feed his 40 cows. 

How long can he feed 30 cows with the same supply of hay?

 

problem 6 answer


15 workmen need 20 days to build a new street.

How many workmen are needed to build the house in 30 days?

 

problem 7 answer


5 caterpillars need 30 days to cut a canal.

How many caterpillars are needed to cut the canal within 50 days? 

 

problem 8 answer


Jane fills apple juice into thirty 0,75-l bottles. 

If she uses 0,5-l bottles, how man bottles can she fill with the same amount of apple juice?  

 

problem 9 answer


A car is going at at speed of 100 mph. It takes 10 minutes to go the shopping center.

How long does it take the same car if it is going at a speed of 80 mph?

 

PDF-Worksheets


instructions  worksheet 

 

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