[functions]

A sample of water has a biological oxygen demand of 100mg/L. In order to support aquatic live its biological oxygen demand must be less than or equal to 5mg/L. By what factor must this water be diluted with pure water in order to support aquatic live?

I attempt to think in this way, but I not sure whether the answer correct or not.
Let x be the factor this water must be diluted with pure water in order to support aquatic live.
Then 100mg/(x L)=5mg/L. x=20.

How is my solution? is it correct?

[Dan]

Looks sensible to me.