Original: When the Count presses about the letter accusing the Countess of infidelity, Susanna and the Countess reveal that the letter was written by Figaro, and then delivered through Basilio.
Simplified: When the Count asks her to explain about the letter accusing the Countess of infidelity, Susanna and the Countess explain that the letter was written by Figaro, and then delivered through Basilio.