After Willy hears Biff's news about Bill Oliver, what flashback occurs?
After Biff tells Willy Loman about the disastrous attempt to meet with Bill Oliver, Willy reverted back to when Biff was younger, and had flunked Math, hence losing the few extra points he needed to enter the University of Virginia. During this argument, he also reverts to when Linda told him that Biff went to see Willy at his hotel, and how Biff found out about the mistress that was with his father at the hotel. Willy also remembered in a flash how Biff cried, how he promised him that he would go see the teacher, and how everything will be OK. Yet, out of all this, the first memory was flunking Math because, had Biff not flunked (according to Willy), his life would have been "set".