(a) 10% decrease
Let the original price of the book be Rs 100.
Decreased price of the book = Rs 75
Increased price of the book after 20% increase
= \(\frac{120}{100}\) x Rs 75 = Rs 90
\(\therefore\) Net change in price = Rs 10 decrease
\(\therefore\) % change = \(\frac{10}{100}\) x 100 = 10% decrease.