How do you determine a pitchers ERA in baseball?

Question

how do you calculate a baseball pitchers ERA?

Answer 1

You take the number of earned runs (Runs not by error, or by base runners that were there by the time you started to pitch) and multiply by 9.

Answer 2

they just do, dont question it

Answer 3

Earned runs given up divided by the number of innings multiplied by 9 gives you Earned Run Average.