What formula is utilized to answer if vehicle A and vehicle

What formula is utilized to answer: if vehicle A and vehicle B leave from the same location, traveling same direction but at different speeds how many miles will it take vehicle B to Catch vehicle A if B is going 120 mph and A is going 100 MPH?

Solution

distance = rate*time
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B DATA:
rate = 120 mph ; distance = x miles; time = x/120 hrs
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A DATA:
rate = 100 mph ; distance = x miles ; time = x/100 hrs
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Note: To have a problem the two vehicles have to leave the
starting point at different times.
Ex: Say that car A leaves 1 hr before car B; you get the following equation:
time A - time B = 1 hr
x/100 - x/120 = 1
120x - 100x = 100*120
20x = 100*120
x = 600 miles
B will catch A at the 600 mile marker.