Hello again!
when he invested $4000 at 5% he increased his income from 0 to 200
adding 4% of 3500 increased it by 140
totaling to $340
he has $2500 left. and he needs $160 more income
160 is 6.4% of 2500
so he needs to invest the remaining $2500 at 6.4% to get the last $160
of his income
I hope this helps, and have a nice day!