A motor car does a journey in 17.5 hours, covering the first half at 30 km/h and the second half at 40 km/h. Find the distance of the journey?

Let the total distance be 2X.

A_____X km_____M_____X km_____B

Total time taken in the journey = 17.5 hours
Time taken to cover X km at 30 km/h = $$\frac{{\text{X}}}{{30}}$$
Time taken to cover X km at 40 km/h = $$\frac{{\text{X}}}{{40}}$$
Now,
$$\eqalign{ & {\frac{X}{{30}}} + {\frac{X}{{40}}} = 17.5 \cr & {\frac{{ {40X + 30X} }}{{1200}}} = 17.5 \cr & 70X = 17.5 \times 1200 \cr & X = 300\,km \cr & {\text{Total}}\,{\text{distance}},\,2x \cr & = 600\,km \cr} $$