The above ngram search would suggest that a one hundred has always been less frequently used in written language and as such should probably be avoided You can see from these examples that 0.01% gap behaves differently across the percentage scale. Your other suggestion of by one hundred times is definitely better than a.
Marking or beginning a century, with the example the centurial years 1600 and 1700 Therefore a is much, much better But there is a word that is widely used to indicate the range of years or centuries covered by an article or book
2 use 100% when you are stating mathematical thought like statistics Is it less than $100 or under $100 Is it more than $100 or is it over $100 Ask question asked 14 years, 3 months ago modified 14 years, 3 months ago
People often say that percentages greater than 100 make no sense because you can't have more than all of something This is simply silly and mathematically ignorant A percentage is just a ratio between two numbers There are many situations where it is perfectly reasonable for the numerator of a fraction to be greater than the denominator.
The second and third examples are both correct Which one you use is mostly a matter of preference, although a hundred appears more frequently than one hundred There is also another form, an hundred, which was common in the past, but has mostly fallen out of use A hundred, an hundred, one hundred:
It will depend on the context Is your 'decade' 10 consecutive years, or is it, say, the noughties Is the 'century' 100 consecutive years, or, say, the 20th century? In a scientific or mathematical context they might occur, in which case you should use standard index form 1×10¹⁰⁰ for one followed by 100 zeros
If soap a kills 100% and soap b kills 99.99% of bacteria, the remaining amount of bacteria after applying a (0%) is infinitely smaller than the remaining amount of bacteria after applying b (0.01%)
