Tony's Velocity

Let the distance between A and B be $d$ .

We know that Speed = Distance $\div$ Time, therefore Time = Distance $\div$ Speed

The time taken to travel from A to B, $t_1 = \dfrac {d}{36}$

The time taken to travel from B to A, $t_2 = \dfrac{d}{48}$

The total time taken is 3 and a half hours. Therefore,

$t_1 + t_2 = 3\dfrac{1}{2}\\ \dfrac{d}{36} + \dfrac{d}{48} = \dfrac{7}{2}\\ \dfrac{4d}{144} + \dfrac{3d}{144} = \dfrac{7}{2}\\ \dfrac{7}{144}d = \dfrac{7}{2}\\ d=\dfrac{7}{2} \times \dfrac{144}{7} = \boxed{72}$

Let the distance from A-B be d.

Because of this, the total distance between A-B then back, is 2d.

It took 3 and a half hours (210 minutes) to travel the distance.

The ratio of the speed from A-B : B-A is

                                            3 : 4   (36 minutes : 48 minutes)

When you add 3 : 4 you get the total ratio 7.

210/7 = 30 minutes

30 minutes (4) = 120 minutes

30 minutes (3) = 90 minutes

120 minutes = The time took to travel from Point A to Point B

90 minutes = The time took to travel from Point B to Point A

90 minutes = 1.5 hours = B:A

1.5 hours (48 km/hr)

                                    = 72

Let t = time to travel from A to B. Then 3.5 - t = time to travel from B to A.

D = R * ;T so D(A-B) = 36 * t = D(B-A) = 48 * (3.5 - t)

36 * t = 168 - 48 * t

84 * t = 168

t = 2

D = 36 * 2 = 72 km