Using the relation between velocity, distance and time, it is found that their average speed changed by -2.5%.
Velocity is distance divided by time, that is:
[tex]v = \frac{d}{t}[/tex]
In this problem:
Then:
[tex]v_2 = \frac{d}{t} = \frac{1.17d}{1.2t} = \frac{1.17}{1.2}v = 0.975[/tex]
1 - 0.975 = 0.025 = 2.5%
Their average speed changed by -2.5%.
To learn more about velocity, distance and time, you can take a look at https://brainly.com/question/24316569