Hi,
Let x represent the truck hauled on its last trip
Question States***
(300 + 350 + x)/3 = 350 tons, the average for all three trips
Solving for x
(300 + 350 + x) = 350*3
650 + x = 1050
x = 400 tons
CHECKING our Answer
(300+350+400) / 3 = 1050/3 = 350tons