If the price of kerosene be raised by 11%, find by how much percent a house holder must reduce his consumption of kerosene so as not to increase his expenditure?

Let the price of kerosene be x and household
consumption be y.
Expenditure = xy
New cost of kerosene = 1.1x, let new household
consumption be z.
In order to not to increase expenditure,
xy = 1.1x * a
⇒ a= 10y/ 11
⇒ consumption should decrease by = 1/11 X 100 = 9.09%