UPSC Prelims 2001 Q114: Indian Economy – National Income & Economic Growth
The following table shows the percentage change in the consumption of electricity by five
towns P, Q, R, S, T from 1986 to 1988:
Town Per cent change from 1986 to 1987 From 1987 to 1988
P +8 −18
Q +15 −11
R +6 +9
S −7 −5
T +13 −6
If town T consumed 500,000 units in 1986, how much did it consume in 1988?
- (a) 371,000 units
- (b) 531,100 units
- (c) 551,100 units
- (d) 571,100 units
Answer: (b) 531,100 units
Explanation
Electricity consumed by town T in 1988
| Step | Working |
|---|---|
| Given for town T | 500,000 units in 1986; +13% from 1986 to 1987; −6% from 1987 to 1988 |
| 1987 | 500,000 × 1.13 = 565,000 units |
| 1988 | 565,000 × 0.94 = 531,100 units |
| Check | Not 500,000 × 1.07; the two changes apply one after the other |
Given: town T consumed 500,000 units in 1986; consumption changed by +13% (1986 to 1987) and then by −6% (1987 to 1988). The percentage changes compound: 500,000 × 1.13 × 0.94 = 531,100 units.
Answer: (b) 531,100 units.
Why each option is right or wrong
- (a) Incorrect. 371,000 does not follow from +13% and then −6%.
- (b) CORRECT. 500,000 × 1.13 × 0.94 = 531,100.
- (c) Incorrect. 551,100 is off by 20,000; recheck the second step.
- (d) Incorrect. 571,100 would need a rise in the second year.
Key point
Successive changes of a% and b% give a net change of a + b + ab/100 = 13 − 6 − 0.78 = 6.22%.
Remember
Successive % changes multiply: 1.13 × 0.94.
Common mistakes
- Adding the percentages (+13 − 6 = 7%) to get 535,000.
- Applying −6% to the 1986 figure, not 1987.
- Reading the row for the wrong town.
Exam tip
Apply successive percentage changes by multiplying factors.
Sources
- NCERT Class VIII — Mathematics, chapter 'Comparing Quantities' (percentage increase and decrease)
- R. S. Aggarwal — Quantitative Aptitude, chapter 'Percentage'