The following (cited at ELU) from the Cambridge Grammar of the English Language still confuses me. I've recast the formatting for readability and discuss only [i] to [iii]; I think I understand the other examples.
The relation between less and fewer is fairly complex. In non-count singulars only less is possible:
Kim has less
fewermoney than Pat. In plural NPs we have: 
i. She left less than ten minutes ago.
ii. Less/Fewer than thirty of the students had voted.
iii. He made no less/fewer than fifteen mistakes.
Both [i] and [ii] have than + numeral. In [i] 'ten minutes' expresses an amount of time rather than
a number of individuated units, and in such cases fewer is virtually impossible—just as few would be
in a comparison of equality: She left as little
fewas ten minutes ago.* Similarly with We paid less than thirty dollars for it; She’s less than forty years old; We were going at less than ten miles an hour.
1. Why are fewer and few wrong? 'ten minutes' DOES express
a number of individuated units, because it expresses an increment of ten minutes, rather than minutes themselves. You can certainly count intervals of 10 minutes, so fewer should be used?
2. Why is few impossible
in a comparison of equality? The above doesn't appear to explain.
In [ii] we are concerned with countable individuals and little cannot be used in a comparison of equality (
as little as thirty of the students); nevertheless, for inequality less is more common than fewer in this construction. The same applies with percentages: Less/Fewer than 30% of the students had voted.
Construction [iii] has the comparative form following no: though the interpretation is count plural, less is here again more common than fewer.
3. The above implies that only few is truly right for [ii] and fewer for [iii],
so why's less 'more common' in both cases?