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?

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

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

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

To find out when Car A and Car B will meet, we need to determine the distance traveled by each car and then calculate the time it takes for the total distances to be equal.

First, let's calculate the distance traveled by Car A:
Distance_A = Speed_A * Time_A
Distance_A = 25 mph * 3 hours
Distance_A = 75 miles

Next, let's calculate the distance traveled by Car B:
Distance_B = Speed_B * Time_B
Distance_B = 30 mph * 6 hours
Distance_B = 180 miles

Now, we need to find the time it takes for the total distances to be equal:
Total distance = Distance_A + Distance_B
Total distance = 75 miles + 180 miles
Total distance = 255 miles

Since both cars are traveling towards each other, they will meet when the total distance traveled by both cars is equal to 255 miles.

Finally, let's calculate the time it takes for the total distances to be equal:
Time = Total distance / Combined speed of the cars
Time = 255 miles / (25 mph + 30 mph)

To simplify the calculation, we need to convert miles per hour (mph) to miles per minute (mpm) using the conversion factor 1 hour = 60 minutes:
Combined speed = (25 mph + 30 mph) = 55 mph
Combined speed = 55 mp min / 60 min/hour

Time = 255 miles / (55 mp min / 60 min/hour)
Time = 255 miles * (60 min/hour) / 55 mp min
Time ≈ 278.2 minutes

Therefore, Car A and Car B will meet approximately after 278.2 minutes.