Question 89312
distance(d) equals rate(r) times time(t) or d=rt; t=d/r and r=d/t

Let r=speed of express
Then r-30=speed of freight train

distance express travels=speed of express times 3 hours=3r
distance freight train travels=speed of freight train times 3 hours=(r-30)*3

Now we know that as they pass the distance express has traveled plus the distance the freight train has traveled equals 390km.  So our equation to solve is:

3r+3(r-30)=390  get rid of parens

3r+3r-90=390  add 90 to both sides

3r+3r-90+90=390+90  collect like terms

6r=480  divide both sides by 6

r=80km/hr--------------------------speed of express

r-30=80-30=50km/hr---------------------speed of freight train

CK
3*80+3*50=390
240+150=390
390=390

Hope this helps----ptaylor