x=first train speed
y=second train speed
sum of speeds is 725.2
x+y=725.2
x is 10.8 more than y
x=10.8+y
subsitute 10.8+y for x
10.8+y+y=725.2
10.8+2y=725.2
minus 10.8 from both sides
2y=714.4
divide both sides by 2
y=357.2
x=10.8+y
x=10.8+375.2
x=386
the speed of first train is 386mph
second train is 357.2mph