Your checking account is overdrawn by $50 you write a check for $20 what is the balance in your account

Starting point: $0.00
You had an overdraw of $50 because that's not what you didn't have any money in your account. 
You then spent more money (by check) to the value of $20.00.
$0 - $50 - $20 = -$70

You overdrawn to the value of $70.

Your balance is negative $70 (-$70).

I hope this helped!!