What is the constant interest rate for a payment period of 3 periods if 10 currency units are paid regularly and the present cash value is 900 currency units.
如果现值为 900 个货币单位的一笔贷款要分三个还款周期还清，而每次的分期支付金额为 10 个货币单位，那么利率应该是多少呢？