If the length of a coaxial transmission line is increased from 20 metres to 40 metres, how would this affect the line loss?

Transmission line loss is directly proportional to the physical length of the cable. If you double the length of the line, you double the amount of material the signal must travel through, which means the total signal loss in decibels also doubles. Increasing a value from 20 metres to 40 metres is a 100% increase in length (you are adding 100% of the original amount). Therefore, the line loss is also increased by 100%. The other percentages represent incorrect mathematical interpretations of doubling a value.