I just did it that way for the 2013 numbers (because it was the fewest and fastest) and that’s what he did.
Sorry sir, but that’s not how ERA works.
Take the top 10 or so guys in innings pitched on each team. Add up all of the innings those guys have thrown (And fyi, when adding 20 and 20.1 it’s actually 20 and 20.33333, and 20.2 would be 20.66666). Save this total innings number.
Then add up the earned runs those same guys have allowed…and multiply that number by 9. Then divide that number by the total innings number to get the combined ERA of those 10 pitchers.