You can use your own car to prove it.

Average speed will be the same for any distance, so I chose 240 miles to work with here:

Just solve this by a simple formula -

$\frac{2ab}{a+b}$

Where a = First speed and b = Second speed

I'll do proving by example.

So for this one, you need a common multiple of 40 and 60. Let's take it's LCM, 120.

At 40mph, you can travel that distance in 3 hours. At 60mph, you can travel that distance in 2 hours.

In short, 240 miles was traveled in 5 hours. 240/5 is 48mph.

This is not exactly a solution but an approximation from which you can easily answer the question:

Please not that the Velocity is calculated from the formula : $\frac{ {\Delta }x} {{\Delta}t}$ now it's clear that the answer is not dependent on the car itself nor it is related to the type of the car.So it's quite obvious that the car travels the distance faster with the speed of 60mph than the speed of 40 mph and thus , spends more time in travelling with speed of 40 than 60,meaning that most of the average time has been spent in 40mph speed and therefore the avg.Velocity must be closer to 40 than 60.The only answer that satisfies what was said above is 48!

A _ _ _ _ _ _ _ _ _ _ _ B

Let the distance from A to B be d

Time taken to travel from A to B @40 mph:

$\frac{d}{t1}$ = 40 mph

Thus, t1 = $\frac{d}{40 mph}$

Time taken to travel from B to A @60 mph:

$\frac{d}{t2}$ = 60 mph

Thus, t2 = $\frac{d}{60 mph}$

Now, average speed = $\frac{total distance}{total time}$ = $\frac{d+d}{t1+t2}$ = $\frac{2d}{60d+40d} \times 40 \times 60$ = 48 mph

Average speed formula: 2 x speed1 x speed2 / (speed1 +speed2) So, 2 x 40mph x 60mph = 4800. 40mph + 60mph = 100. therefore: 4800/100 = 48 mph average speed.