How to make MEM 10% FBS; 1% P/S - (Oct/28/2021 )

Hi, so I'm trying to figure out the math behind 500ml of MEM w/ 10% FBS; 1% P/S, I looked around the forum and found a similar topic, the answer that was provided was:

" If a bottle of DMEM/RPMI is 500ml and you need to make up a 10% FBS/FCS solution .....then you need to add 55mls of FBS/FCS to the 500ml of media. If you do not then you are adding a 10% error into your experiment straight away."

can someone please explain how they got the 55ml? I was told to divide 500/9, I did get the 55.5, however why is it divided by 9? I am very new to this is and is a bit confusing.

Thanks in advance! 

If you add just 50ml then the final volume will be 550ml and the fbs won’t be 10%. If you add 55ml then the final volume will be 555ml, only slightly below 10% (55.5ml brings you even closer).

when you add both the fbs and p/s you’ll have to take the final volume into consideration