GMAT Quantitative Reasoning Practice Test 4

Average speed can be found by dividing the total distance by the total time. If the journey back took twice as long then the car traveled at half the speed, thus 24-mph. Take the time the car traveled from San Diego to San Francisco as T, and the time it took to get back as $2\mathit{T}$ . The total distance is $48\mathit{T}+24\times 2\mathit{T}=96\mathit{T}$ , now divide it by the total time, 3T to get 32 mph.