Question 1128773
.
<pre>
Let x be the speed of the train, in km/h.


Then the speed by the car is (x+50) km/h.


The full distance equation is


    4x + 3*(x+50) = 640       <<<---=== notice that the time to travel by the car is  7-4 = 3 hours


    4x + 3x + 150 = 640


    7x = 640-150 = 490  ====>  x = 490/7 = 70.


<U>Answer</U>.   The speed of the train is  70 km/h.
</pre>

Solved.