race distance : d

N's speed : v
J's speed : v+20


J's race
d/12 = v+20
d = 12v + 240


N's race
2/3 . d/12 = v
d = 18v


18v = 12v + 240
v = 40 cm/s

d = 18x40 = 720 cm


...

Non algebra method :

1 - ⅔ = ⅓

Jupiter travelled ⅓ more of the race distance than Neptune.


20cm/s

Every second , Jupiter is 20cm faster than Neptune. This also means Jupiter travels 20cm more than Neptune every second.

So after 12 seconds, when he crossed the finish line,

20cm x 12 = 240cm
Jupiter travelled 240cm more than Neptune in total.

Now 240cm is ⅓ of the race distance.
So the race distance is just 240cm x 3
= 720cm (or 7.2m)

Thank you very much

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