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| Direct and Inverse Proportion |
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Direct Proportion
If one quality is directly proportional to another it changes in the same way.
As it increases, so does the other – as it decreases, the other decreases also.
Example: The cost of sweets is directly proportional to the number of sweets bought.
If 1 sweet costs 10 c, 5 sweets cost 5 x 10 = 50 c.
or
If 10 sweets cost 60 c, 1 sweet costs 60/10 = 6 c.
Method
For direct proportion, we find the value of 1 by division and then multiply to find the total value.
For example, a car uses 20 litres of petrol in travelling 140 km. How much would be used on a journey of 35 km?
1 km = 
35 km = x 35 = 5 litres
Rule: Divide to find 1 and then multiply.
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Inverse Proportion
If one quantity is inversely proportional to another, it changes in the opposite way – as it increases, the other decreases.
Example: If 8 men take 4 days to build a wall, how long would it take 2 men (assuming they work at the same rate)?
First, we decide whether the problem is direct or inverse proportion.
In this case, if less men are used, they will take longer, so it is inverse proportion.
Method
8 men take 4 days
1 man takes 8 x 4 = 32 days
2 men take  = 16 days
Again we find the value of 1 by multiplying. Then divide to find the final answer.
Note: This process is the opposite of direct proportion.
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