有道题怎么算和答案都不一样 望大神赐教。。
原体是这样得  a  loan of 20000 is repaid with varying payments at a continuous rate of 1000+200t, Interest is credited at a corce of interest of f(t)=1/(5+t). Find the time that at which the loan is paid off.

我得思路是loan ＝ 所有payment 得present value  。。
然后列一个intergral得式子求n

我算出来得数是20  正确答案是40 ＝ ＝不知道错哪里。。。
望大神赐教