Combined work uses rates, not times. Add what each person does in one hour, then invert to get the time together. If A finishes in 6 hours and B in 3, together they do 1/6 + 1/3 = 1/2 of the job per hour, so two hours. Rate problems as a set are in rate problems.
One hour of work
Write each rate as a fraction of the job per hour. Add the fractions. The reciprocal is the time together. If someone leaves after two hours, subtract the unfinished fraction and finish with the remaining rate.
Travel still uses distance equals rate times time, in d = rt. Percent change is a different formula, in percent change. The short list is in ACT Math formulas.
The bottom line
Add hourly rates. Invert the sum. Do not average the two times.
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