You can put this solution on YOUR website!
Okay let's see what the ratio really means.
for every 4 km/h that the fast car accelerates, the slower car only accelerates 3 km/h.
Looking at the fast car:
So one "unit" in our ratio is 20 km/h. So for the fast car to do 4 units (80 km), the slower car only accelerates up to
in 9 seconds.
Now, if it took the slow car 9 seconds to get up to 60 km/h, this means it took him 3 seconds to accerlerate 20 km/h. This means that it will take him another 3 seconds on top of the 9 seconds (since 60+20=80). So it will take the slower car 9+3=12 seconds to do the same amount of acceleration.
I hope this helps a bit..
Let me know if you are still unsure!:)