If Rs.10 be allowed as true discount on a bill of Rs.110 at the end of a certain time , then the discount allowed on the same sum due at the end of double the time is

Present worth  = Amount - True Discount = 110 -10 = Rs.100 

SI on Rs.100 for a certain time = Rs.10

SI on Rs.100 for doube the time = Rs.20 

True Discount on Rs.120 = 120 - 100 = Rs.20 

True Discount on Rs.110 =  $110\times \frac{20}{120}$ = Rs.18.33