A balance holds 20 units, and a payment of 50 units is taken from it. What is the new balance?

Explanation

The opening 20 units of the payment empty the balance, and the remaining 30 units are still owed, so the balance ends 30 units below zero. Answering 30 records the size of the shortfall as though it were money still held, and adding the 20 and the 50 describes a payment that never happened.