The counting numbers have two forms called cardinal and ordinal . One, two, three, four, etc. First, second, third, fourth, etc.
Every language has both cardinal and ordinal numbers.
5 is the first multiple of 5. 5 is a part of 10, a part of 15, and so on. Since 15 is the third multiple of 5, we say that 5 is the third part of 15. We use that same ordinal numbers to name the part. 5 is the fourth part of 20, the fifth part of 25, and so on.