Question 506913
Let
Total Length of the Journey = x
Completed Journey = 3600
30% of x = x * 30% = x * 30/100 = x * 3/10 = 3x/10

Total Length of the Journey = remaining Journey  + Completed Journey (30% of x)
x= 3600 + 3x/10
Multiply by 10 both sides of the above equation
10x=10 * 3x/10 + 10 * 3600
10x=3x + 36000
10x-3x=36000
7x=36000
Divide by 7 both side of the equation
7x/7=36000/7
x= 5142.857

Total Length of the Journey = 5142.857 miles

Check
Train Completed 30% so 70% still remain
 
Remaining 70% of Total Journey = 70/100 * 5142.857
Remaining 30% of Total Journey = 7/10 * 5142.857
Remaining 30% of Total Journey = 7 * 514.2857
Remaining 30% of Total Journey =3599.9999 miles (Round it off)-->  3600.00 miles