UPSC 2017 CDS II Math Paper

Expenditure = Price × Consumption
Let p and q be the initial price and consumption respectively.
Suppose the consumption afterprice rise be Q.
∴pq=(p+

12

100

p)Q⇒Q=

100

112

q
Therefore, percentage reduction in consumption
=(

q− 100 112 q

q

)×100%
=

1200

12

%=

75

7

%=10

5

7

%
Trick:
If the price of a commodity increases by k%, then to keep expenditure constant, decrease in consumption is

k

(100+k)

×100%
∴ Reduction in consumption
=

12

(100+12)

×100%
=

12

112

×100%=

75

7

%=10

5

7

%