Answer:
a) Abby's expression

Renato's expression,

b) Both methods will give the same answer.
Step-by-step explanation:
a) Let x be the number of miles,
∵ Double of x = 2x,
20% of 2x = 
Thus, number of kilometers in x miles = 2x - 20% of 2x
= 
Which is Abby's expression,
Now, Renato's expression of kilometers in x miles = 

b) 
Hence, two methods will give the same answer.