Muhammad's pennies

Jenny places 100 100 pennies on a table, with 30 30 showing heads and 70 70 showing tails. She chooses 40 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?

This problem is shared by Muhammad A.

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