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Direct and inverse proportion
Direct proportion
- If two quantities are in direct proportion, when one quantity increases, the other also increases by the same percentage.
inverse proportion
- Inverse proportion is when one of the value increases, the other value decreases.
Direct proportion example
- Method :
- For example, a car uses 20 litres of petrol in travelling 140 km. How much would be used in a journey of 35 km?
= 1km = 20/140
= 35 kms = 20/140 x 35 = 5 litres
= Rule: Divide to find one and then multiply.
Inverse proportion example
If one quantity is inversely proportional to another, it changes in the opposite way — as it increases, the other decreases.
= 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.
= 1 man takes 8 x 4 = 32 days
= 2 men take 32/2 = 16 days
= Again we find the value of one but by multiplying. Then divide to find the final answer.
Note: this process is the opposite of Direct Proportion.
- Example:
= 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:
= 1 man takes 8 x 4 = 32 days
= 2 men take 32/2 = 16 days
= Again we find the value of one but by multiplying. Then divide to find the final answer.
Note: this process is the opposite of Direct Proportion.
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