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If the price of tea is increased by 20%, then by what percentage must a householder reduce her consumption of tea so as to leave her expenditure on tea unchanged?
20%
25%
3331тАЛ%
1632тАЛ%
1632тАЛ%
Use the fraction conversion: 20% increase is a ratio of 1/5. To keep expenditure constant, the consumption must decrease by the ratio 1/(5+1) = 1/6, which is 16 2/3%.
Price of tea is increased by 20%, and the total expenditure is to be kept constant.
Reduction┬аPercentage=(100+xxтАЛ)├Ч100
Use the fraction conversion: 20% increase is a ratio of 1/5. To keep expenditure constant, the consumption must decrease by the ratio 1/(5+1) = 1/6, which is 16 2/3%.
Students often wrongly assume the reduction should be 20%, forgetting that the base price has increased, making the denominator larger.
Establish the relationship
Since Expenditure = Price ├Ч Consumption and expenditure is constant, the ratio of Consumption is inverse to the ratio of Price.
E=P├ЧCтЯ╣CтИЭP1тАЛ
Calculate the price ratio
Initial price is 100, new price is 100+20=120. The ratio of Price becomes 120:100=6:5.
P1тАЛP2тАЛтАЛ=100120тАЛ=56тАЛ
Determine consumption reduction
The consumption must change from 6 units to 5 units. The reduction required is 1 unit out of 6.
Reduction=66тИТ5тАЛ=61тАЛ
Convert to percentage
Multiply the fraction by 100 to get the percentage value.
61тАЛ├Ч100=1632тАЛ%
D is correct because a 20% price increase requires a 16 2/3% reduction in consumption to keep expenditure constant.
This concept is identical to the 'speed-time-distance' relationship where distance is constant, speed and time are inversely proportional.