No, I'd parse that as being 2/5. The difference is, you didn't say "40% more sick days are taken on Monday and Friday", which would imply (M+F) = 1.4(T+W+T) ->

No, I'd parse that as being 2/5. The difference is, you didn't say "40% more sick days are taken on Monday and Friday", which would imply (M+F) = 1.4(T+W+T) -> 58% of sick days taken on Monday and Friday, and 42% of sick days taken on the remaining days.