welcome to cell culture & dimensional analysis
125/50µl or 2.5 cells/µl is your concentration (better way of expressing it)
2.5 * 8000µl = 20,000 total cells you have. 1000µl in 1ml that's why the 8000µl
20,000 cells / 1000 cells/ml = 20mls final culture volume to reach desired concentration
20mls - 8mls = 12mls so you need to add 12mls of medium / saline / whatever to your 8mls to dilute it to the correct concentration.
when you ask about solvent to ADD to make 5mls. Prob with this question is you don't state the desired concentration. So assuming its still 1000 cells/ml kinda go backwards,
1000 cells/ml * 5mls = 5000 cells is what you want to get
to get it:
5000 / 2.5 cells/µl = 2000µl or 2mls of your culture.
So take 2mls of your culture and mix it with 3mls of solvent / medium (5mls - 2mls).
I'm quite certain chris below is not correct.