Answer: (b) \(Rs.545.445\)Solution: Given, selling price (SP) = \(Rs.500\)profit percent = \(10 \ \%\), then $$ SP = \frac{100 + P \ \%}{100} \times CP $$ $$ CP = \frac{500}{100 + 10} \times 100 $$ $$ = \frac{500}{110} \times 100 $$ $$ = Rs.454.54 $$ \(20 \ \%\) of the cost price, $$ = \frac{454.54 \times 20}{100} $$ $$ = Rs.90.908 $$ Hence, to gain \(20 \ \%\) the book should be sold at Rs. $$ = 454.54 + 90.908 $$ $$ = Rs.545.445 $$