Because the Destiny Knot passes down 5 of the parents' stats at random, the best chance that you'll get a 5IV Dratini would be if the stat that is not passed down is the worst stat of the parents (in this case, SpA), which is a 1/6 chance, or 16.667%.
Honestly, you could probably stop here, but technically, for the 5/6 chance that one of the other stats is not passed down instead, there is still a 1/32 chance that the stat randomly rolls a perfect 31. For this, we multiply 5/6 by 1/32, which gives 5/192. You could get a 5IV either by the 1/6 chance or by the 5/192 chance, so for our final probability, we add them together for a 37/192 chance, or 19.27%.
