4 * 42 + 55 * x / ( 4 + x ) = 50.
The answer of x comes out to be 6.4. Which means that the truck will have to travel around 6.4 miles at an average speed of 55 miles to make it 50.
Well to understand how i constructed its equation is very simple. I equated the total average speeds answer to 50 and just calculated the average speeds with the help of algebra where x is the number of miles at 55 mph.
