Etienne, I hope I'm not teaching my grandmother to suck eggs but there is a general rule for when to use 'less' and when to use 'fewer':

if you can count the number of things, use 'fewer'

if you can't count them (numerically), use 'less'

i.e. No fewer than 10 people came to Ed's leaving party
There are fewer dogs than cats in this photo
(dogs, cats and people can be counted)

But:

Use less flour next time
I have less faith as I grow older

(flour and faith can't be counted, as such)

KK's example is based on the fact that the first example refers to the number of difficult questions (a countable thing) so it's 'fewer'
But the second example refers to the level of difficulty of the questions (a non-countable thing) so it's 'less'