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Let's write distances:
Day 1 :  A1 = B1 - 117    (1)
Day 2 :  A2 = B2 + 185    (2)
Day 3 :  A3 = B3 -  48    (3)
-------------------------------  Add equations (1), (2) and (3).  You will get
        A1 + A2 + A3 = B1 + B2 + B3 + (-117 + 185 - 48),     or
        A1 + A2 + A3 = B1 + B2 + B3 + 20
It means that in three days Truck A covered the distance 20 miles longer than Truck B.
In other words,  Truck A is 20 miles ahead of Truck B after 3 days driving.    ANSWER
Solved.
It is a case and a good example when the Algebra language works better and is more clear than a human language.