If the price of coffee is raised by 25%, by what percentage a householder must reduce his consumption of coffee so as not increase his expenditure? If the price of coffee is raised by 25%, by what percentage a householder must reduce his consumption of coffee so as not increase his expenditure?

Price of the coffee is $x $  Expenditure is $y$   Consumption will be $xy$
Increased price of coffee is $1.25x$   Expenditure is y  consumption will be $1.25xy$

Now what to do next?
Hint:

If $p$ is the old price and $q$ is the old consumption, the new consumption $x$ must be such that:
$$
1.25p \cdot x=p\cdot q
$$

so: $x=...$



the percentage variation of consumption is:
$$
\Delta( q)\% =\frac{x-q}{q} \cdot 100=\left(\frac{x}{q}-1  \right)\cdot 100
$$

with ( from the previous equation)

$$x=\frac{q}{1.25}$$

can you do this?
Since exp. Constant

P1C1=P2C2

P1C1=5/4P1C1

C1/C2=5/4

(C1-C2)*100/C1%=1/5*100%=20%