posted by anonymous on .
Jenny places 100 pennies on a table, with 30 showing heads and 70 showing tails. She chooses 40 distinct pennies uniformly at random and turns them over. That is, if a chosen penny was showing heads, she turns it to show tails; if a chosen penny was showing tails, she turns it to show heads. After this process, what is the expected number of pennies showing heads?