Math question

I hope someone out there in nursing land can help me understand this math problem from a dosage calculation book I'm working with

A package insert reads as follows:

"Reconstitue as follows: An initial concentrate of 5mg amphotericn B per mL is first prepared by expressing 10mL Sterile Water for injection.......The infusion solution, providing 0.1mg amphotericin B per ML, is then obtained by further dilution (1:50) with 5% Dextrose injection."

Now I understand that for the initial concertration 10mL of sterile water is added. What I don't get is that the book says " to further dilute the solution containing 5mg/mL to 0.1 mg/mL, by adding 1 mL (5mg) of solution to 49 mL of 5% Dextrose and Water Injection for a 1:50 dilution". How was that 49 mL of 5% Dextrose and Water derived? Where did the book get 49 mL from? How was 49 mL arrived at? It must have something to do with (1:50) but I'm not getting it.

Please explain someone.

Mahalo.