Answer
$26.8 \%$ of those who minimised or decreased their spending on music were internet users.
Work Step by Step
According to Bayes' theorem:
$P(I|D)=\dfrac{P(D|I)P(I)}{P(D|I)P(I)+P(D|I')+P(I')}~~~~~~~~(1)$
Here, we have
$P(D|I)= 11 \% =0.11\\ P(I)= 40 \% =0.4\\ P(D|I')=20 \% =0.2$
Now, we will now use formula (1) and the given data to obtain:
$P(I|D)=\dfrac{P(D|I)P(I)}{P(D|I)P(I)+P(D|I')+P(I')}=\dfrac{(0.11)(0.4)}{(0.11)(0.4)+(0.2) (1-0.4)}$
or, $=0.268$
or, $=26.8 \%$
Thus, we conclude that $26.8 \%$ of those who minimized or decreased their spending on music were internet users.
