This is a suggested solution to the GCE O Level Physics question in November 2011, Paper 1, Multiple Choice question 4.
The key concept to know for this question is Average Speed. By definition, the formula of average speed is:
The question said that a car driver takes a total of two hours to make a journey of 75km. Further information provided is that she takes a break for half an hour and spends a quarter of an hour stationary in a traffic jam. In other words, she is stationary for three quarter of an hour. This means that she has only one and a quarter hour to travel the remaining journey of 75km. For this question, the student is required to compute the average speed of the driver during the rest of the time for the whole journey to complete in two hours.
Using the formula of average speed,
Average Speed = (75km) / (1 1/4 hr) = 60k/h