Another nice try, soot, but not correct.

6. Are the snails equidistant from each other, an/or do the last two perhaps stop to chat while waiting to cross?
Third time lucky mayhap (and I think one of five options all of which seem to work mathematically)
... and to cover the final option which I can see (unless we are using fractions/decimals )

I think that at this point I'll give you the answer and explanation.

The easiest way is to set up a proportion. It takes 18 "steps" for all of the snails to cross. It takes 9 steps for one snail to cross.
18/50 = 9/x
25 = x

Wow Pandora, I must admit that I am totally confused on this one. I don't know where the steps come from

edit:
I imagine they were a way of definging time for the ratio.

These are great Brain Ticklers!

edit #2:

So your solution means each snail is on the track for 25 minutes?

5

In 6, I follow the idea of 18 "steps" for all the snails to cross, also that if 18 steps take 50 minutes, 9 must take 25 minutes, but somehow that seems to defy logic if I am trying to visualise a line of snails in the process of crossing...

Spot on, soot!