![]() All these quantities are described with respect to one parameter, that is time. The proper description of motion was given by Sir Isaac Newton. Following this logic, and knowing that time is a crucial component of average speed, we expected our answer to be closer to 40 than to 60.The term motion can be described by using some physical quantity terms like speed, velocity, distance, displacement, and acceleration. ![]() Performing a fast check will help eliminate silly mistakes.Īt the beginning of the problem, we drew the conclusion that Joe spent less time traveling at 60 miles per hour than he did at 40 miles per hour. ![]() Since average speed problems involve multiple speeds, times, and distances, test-takers must make sure they haven’t mixed up any values and that their answer seems reasonable. In the steps above we calculated total distance (120 miles + 120 miles = 240 miles), and we just found total time (5 hours).Īverage Speed = 240 miles/5 hours = 48 mphīefore selecting this answer, it’s helpful to check for any errors. It took Joe 2 hours to drive 120 miles at 60 mph, and it took 3 hours at 40 mph, giving us a total time of 5 hours. Time spent driving 40 mph = 120 miles/40 mph = 3 hoursĪs we said at the start of the problem, it will take less time to drive 120 miles at 60 mph than it will to drive that same distance at 40 mph, and we were right. Time spent driving 60 mph = 120 miles/60 mph = 2 hours Joe drives at two different speeds: 60 mph and 40 mph. We’ll use the formula: time = distance/speed The total distance is easy to compute: first 120 miles, and then another 120 miles, which gives us a total distance of 240 miles. Joe drives 120 miles at 60 miles per hour, and then he drives the next 120 miles at 40 miles per hour. We’ll have to calculate total distance and total time in order to solve for average speed.Įven though we need to make several calculations, everything that we need is in the question text. ![]() Let’s take a closer look at the average speed formula.Īverage Speed = Total Distance/Total Time Because less time is spent driving at 60 miles an hour, the average can’t be as simple as adding 40 + 60 and dividing by two. It should be clear that driving 120 miles at 60 miles per hour will take LESS TIME than driving that same distance at 40 miles per hour. Even before we start the math, let’s stop and think about the given information. Speed represents total distance divided by total time, and this fact completely changes our approach to an average speed problem. We typically find the average of two values by adding the numbers and dividing the total by two, but this doesn’t work when we talk about speed. What’s the Correct Approach to Average Speed Questions? Choice C is NOT the correct answer, and this kind of trick question is a common trap for average speed problems on the GRE. It’s tempting to follow the normal logic of taking averages: add 40 + 60 to get 100, divide by 2, and get C: 50. ![]() What is his average speed for the entire trip in miles per hour? These questions can be tricky if you haven’t practiced solving them, because the most obvious answer choice is a trap. Average speed problems are common on the GRE. ![]()
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