# Is saying "decrease by two-fifths" grammatical?

Suppose there were 100 men in the room, and then 40 left. Is the following sentence natural?

The number of men in the room decreased by two-fifths.

• Are you being asked to speak of the exodus in precise mathematical terms and fractionally in terms of the starting number? You haven't said why you couldn't say "Forty of the men left". And unless you're being asked to describe it in fractional terms, it's rather "unnatural".
– TimR
Mar 23 at 14:03
• In the uk fractions are more commonly used in conversation than percentage. Mar 23 at 17:51
• @JudithJones: I think it depends on the fraction. With small denominators (thirds, quarters, maybe up to tenths) yes, but otherwise I'd expect to hear decimals or percentages. Mar 23 at 18:13
• @KenAdams In most casual conversations we tend to estimate. Nearly half. About a third. Most. Relatively few. A small number. Three-eighths, say, would be unnatural in a casual context
– TimR
Mar 23 at 19:00
• It depends on what fractions in daily conversation. Not all fractions were born equal. Mar 23 at 20:40

It's grammatical, but less natural than the obvious "decreased by 40%".

Yes you can correctly say "decreased by two-fifths." Unless they really need to convey precise information, a native speaker would be more likely to round up "The number of people in the room almost halved."

In a maths lesson or when talking scientifically one could reasonably say "decreased by two-fifths."

• What about when you write an IELTS essay, where you have to summarize the information given in a chart? Mar 23 at 12:47
• That depend on whether you want to convey precise information (like in a maths lesson or like when writing scientifically) or not. For the requirements of the test please consult the test specification (but since this is a test of English, they probably don't care about if you are mathematically correct or not,) Mar 23 at 14:34
• If you want to precisely convey the information in a formal essay. decreased by 40% might be best. Mar 23 at 18:05
• The natural English. we speak will differ from that used in a formal piece of writing. Mar 23 at 18:07
• One hazard, more commonly seen with percentages: Make sure it's clear which number the ratio is taken relative to. It's not uncommon, especially when talking about values which can be expressed in multiple units (speed versus time taken), for people to get confused about that. 50% faster cuts the time by 33%, and "a 50% improvement" could be either if not clearly specified. Mar 25 at 0:31

The sentence "The number of men in the room decreased by two-fifths" is technically correct, but it might not be the most natural or intuitive way to express the situation. It accurately conveys the mathematical reduction in the number of men, but it could be stated more simply and clearly.

A more natural way to phrase it could be:

"The number of men in the room decreased by 40; now there are 60 men left."

This directly communicates the change in the number of men without using fractions or percentages.

• The advantage of this is that it communicates your information to everyone, including people who don't understand fractions or percentages very well. Mar 23 at 18:10
• This would be a great approach when trying to communicate health information to as many people as possible, for example. Mar 23 at 18:11