The time point can be chosen as 0 for the start. The fast car will travel 50 miles more than some d distance which the slow car will travel. The slow car already spent some 50 miles, but that much can now be ignored.
Time will be equal for the slow and the fast car; NOT for their whole trip but just starting from the same point in time. The time spent while slow car went first 50 miles is ignored here.
rate time distance
SLOW 60 t d
FAST 75 t d+50
When this much as clearly understood, you are ready for the next part of the process.
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wait until you understand
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Uniform rates travel rule to relate rate time distance is RT=D.
Solve the system.