We're launching a balance transfer offer: 0% APR for 12 months with a 3% transfer fee, and the average transferred balance is $5,000. Cost of funds is 5%, and the annual loss rate is 2% during the promo. After the promo, APR goes to 20% and the loss rate rises to 4%. Looking at a two-year horizon only, what share of the balance do we need to retain in year two to break even?

我们推一个余额转移产品：前12个月0%利率，收3%转账费，平均转入5000美元。资金成本5%，促销期年损失率2%；促销结束后APR变成20%，损失率升到4%。假设只看两年，第二年至少要留住多少余额才能回本？