delay of 4-1=3 hrs(excluding one hour of breakdown) in first condition is due to the train covering the distance after the breakdown delay of 1 hour at the 3/4th of the original speed------1
however this delay of 3.5-1(excluding one hour of breakdown)=2.5 or 5/2 hrs is due to the train covering 150 kms less distance as mentioned in equation 1 above, at 3/4th of the original speed---2
In 2 , delay is 1/2 an hr less which is due to 150 kms less distance travelled at reduced speed than in 1.
so 150/0.75*s - 150/s=1/2
solving s=100 km/hr ,original speed of train.
now using this value of s in any of the above two equations, d=total distance of journey can be found, for eg. using 1
(d-300)/0.75s - (d-300)/s =3 --------3
{here we are formulating the time of delay 3 hrs, as a result of the difference in time, when the distance travelled after breakdown in the first condition is travelled at the reduced speed to when the distance would have been travelled at the original speed s=100 km/hr if fault hadn't occurred.}
solving 3 will give d=1200 kms