The concept is extremely simple : Distance = Speed * Time
The place where you need to be careful is to make sure that the units that you use for the different quantities have integrity i.e., if the unit for speed used is km/hr, then you should use hour as the unit for time.
Here is a simple question in speed time distance.
Traveling at 4/5ths of her usual speed, Sonal reached office 20 minutes late. How long did she take today?
1. 60 minutes
2. 80 minutes
3. 100 minutes
4. 64 minutes
5. 96 minutes
She travels at 4/5th of her usual speed.
Let her usual speed be 's' km/min. So, today she travels at 4/5s.
Let the usual time she takes be 't' minutes. So, today she will take (t + 20) minutes.
As the distance traveled in both the cases is the same, we can equate them
i.e., st = 4/5s (t + 20)
dividing both sides by 's' we get t = 4/5(t + 20)
or 5t = 4t + 80
or t = 80 minutes.
t is the usual time take.
The time taken today is t + 20 = 100 minutes.
Note: 1. We used km/min as the unit for speed as the unit of time given in the question is in minutes.
2. we did not stop at finding the value of 't'. The question was about finding the time taken today = t + 20.
You can get additional questions on speed time and distance at this link.