Extracted this from Times Magazine,and I am confused by the use of " about 40 percent of whom". Just couldn't seem to get this use of language.Would anyone explain to me this? And sometimes I read " of which", I often end up facing the same confusion.
Original text as below:
As a philanthropist.Bacall has raised more than $10 million for his non-profit,which supports refugees,and as the CEO of Elite Hospitality Group,a development and management company,he employs more than 1000 people,about 40 percent of whom he estimates are foreign-born.
Thank you for helping me.