If I travel by bus, I reach my office 15 min late, and if I travel by car, I reach 10 min early. If the distance between my home and my office is 25 km, then the difference of the reciprocals of average speeds of the bus and the car, in second per metre is:

Distance = 25000 m

Travelling by bus makes you 15 minutes late : t+ 15

Traveling by Car makes you 10 minutes earl : t-20

Total time difference between the bus and the car : 15 - (-10)  = 25 minutes

Convert time difference to seconds: 25*60 = 1500 seconds

Average speed : $$\frac{d}{t}$$

Reciprol of the Average speed : $$\frac{t}{d}$$

Difference of the reciprol of the speeds of the bus and that of the car:$$\dfrac{T_b}{d}-\dfrac{T_c}{d}=\dfrac{T_b-T_c}{d}$$

Substitute the values into the formula:

Difference : $$\dfrac{1500s}{25,000m}=\dfrac{3}{50}$$