| Number (Intermediate) - Direct and Inverse Proportion |
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| Direct Proportion | 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 1: The cost of sweets is directly proportional to the number of sweets bought. If 1 sweet costs 10p, 5 sweets cost 5 x 10 = 50p or If 10 sweets cost 60p, 1 sweet costs 60/10 = 6p Method For direct proportion, we find the value of one 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 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. |
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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 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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