Mathematics · Time and Work

There is a group of persons in which each person has the same work efficiency. A work is done with this group as follows: on the first day only one person worked, on the second day only two persons worked, on the third day only three persons worked, and this pattern is followed for the subsequent days. Following this pattern, they completed the work in exactly 15 days. Find the number of days required to complete the same work if 15 persons work together every day.