減る and 減らす: a lower count and reducing something
Compare a count falling with reducing it, and distinguish the amount of decrease from the resulting count marked by に.
Report a lower count
今月は利用者が減りました。 says that the number of users fell this month. 減る, read へる, reports a quantity change without specifying its cause.
参加者は二十人から十五人に減りました。 means that attendance fell from twenty to fifteen. 十五人に gives the resulting count, not the amount lost.
Choose what to reduce
電気代を減らすため、使わない照明を消しました。 says that someone turned off unused lights to reduce the electricity bill. 減らす, read へらす, marks the target of reduction with を.
塩の量を半分に減らしました。 means that the amount of salt was reduced to half. Preserve the result 半分に rather than interpreting it as removing all salt.
The decrease and the verb forms
参加者が五人減りました。 says that attendance decreased by five people. It describes the same amount of decrease as the earlier change from twenty to fifteen.
減る has the te-form 減って, while 減らす has 減らして. Do not form 減て merely because 減る ends in る; learn 数が減っている and 量を減らして as usable phrases.
In 二十人から十五人に減りました, did the number decrease by fifteen people?
Show the answer and why
No. Fifteen is the resulting number; the decrease is five. The change can also be expressed as 五人減りました.