Goodness of Fit - estimation of parameters - Exponential Distribution

Ms Patel asks her students to record the number of cars passing the school gates for a 40-minute period to use for a project in Statistics. As Ms Patel has done this with her class every year for the last 6 years, she decides to do an investigation of her own and records the time (in minutes) between each car passing instead. The table below shows her results
Time in minutes 0 ≤ t ˂ 2 2 ≤ t ˂ 4 4 ≤ t ˂ 6 6 ≤ t ˂ 8 8 ≤ t ˂ 10 10 ≤ t ˂ 40
Frequency 21 6 0 2 1 1
Using a Chi-squared suitableness of fit test, investigate Ms Patel’s belief that the times between cars passing the school can be modelled by an exponential distribution.