Question 42823
(a) If the car has already traveled 50 mph, then it is this many miles away, in x hours after the second car  has left:

{{{50 + 50x}}}

While the second car:

{{{70x}}}

The distance must be equal when they meet, so we have {{{50 + 50x = 70x}}}

*[invoke explain_simplification "50 + 50x = 70x" ]

So, the time the second car took is really 2.5 hours.

For b, just substitute 70x for x = 2.5 and you'll get 175 miles.