Seven million two thousand five hundred is 7,002,500
Neither cause there both at the same constant rate so one train wouldn’t be traveling at a greater speed unless there is something missing from the question being asked like the actual speed of each individual train then there would be one train traveling at a greater speed
5x+2=47 subtract 2 from both sides
5x=45 divide both sides by 5
x=9
Suppose you add x liters of pure water to the 10 L of 25% acid solution. The new solution's volume is x + 10 L. Each L of pure water contributes no acid, while the starting solution contains 2.5 L of acid. So in the new solution, you end up with a concentration of (2.5 L)/(x + 10 L), and you want this concentration to be 10%. So we have

and so you would need to add 15 L of pure water to get the desired concentration of acid.