Question

for a question like: car A was traveling at 25miles/hour for 3 hours. Car B was traveling at 30mph for 6 hours. When will they meet?

Answer 1

wouldn't it depend on the starting points, ie, how far apart.

Answer 2

distance = rate * time
One way is to make a table and fill it in. The same approach works for doing mixture problems. Your example does not work as you made it up because if they both start together car B will always be ahead.
Let me change your example as follows:
Car A and Car B are 100 miles apart and start toward each other.
Car A goes 20 mph
Car B goes 30 mph
where and when do they meet?
car # rate time distance
A______20__*__t___=___20 t
B______30__*__t___=___30 t

Now since the total distance they rolled was 100 miles you can see that
20 t + 30 t = 100
so
50 t = 100
t = 2 hours
and car A went 20*2 = 40 miles
and car B went 30*2 = 60 miles

Answer 3

Now to make it harder, keep everything the same but say car B starts 1/2 hour later than car A. Then:
car #_ rate __time_____ distance
A______20__*____t_____=___20 t
B______30__*_(t-.5)___=___30 (t-.5)

Now 20 t + 30(t-.5) = 100
20 t + 30 t - 15 = 100
50 t = 115
t = 2.3 hours for Car A
Now our chart is:
car #_ rate __time_______ distance
A______20__*____2.3_____=___46 miles
B______30__*(2.3-.5)=1.8=___54 miles