Fascinating story! Of course there is nothing wrong with your computation, given that you want to express the difference between the racers in seconds per race. The question is: what did the teacher and your son do to get to zero.
You said your son managed to get 0 as an answer, so clearly the best way to get the answer to that queston is by asking him to show you again how he did the computation!
In the mean time we can speculate...
I can think of two possibilities.
- They don't want to express the difference in seconds per race, but as an actual speed. That means: as races per seconds, as calculated by Mike above, or in a somewhat more natural sounding unit, like meters per seconds. Of course the latter is only possible if the numbers of meters in the racing track is given! Suppose they are running over 100 meters, then the difference in speed is 0.219 m/s (obtained by multiplying Mike's answer by 100) which could be rounded down to zero.
This theory has some disadvantages: it requires more advanced math, it requires more information than you said there was and computing actual speed and then rounding it to the nearest integer is an odd way to talk about racing which is rarely seen at the Olympics for instance.
On the other hand: the question asks 'how much faster', not 'how much earlier' which suggests we are looking for speeds rather than time differences.
- There is a mistake: both racing times were first rounded to the nearest integer (18 seconds in both cases) and THEN substracted. This would be WRONG. In fact this is a textbook example of why one should only do the rounding at the very end, after ALL other operations have been carried out.
Perhaps there are more scenario's, I'd like to read them.