Its a bit late to answer, but basically what you need to do is :

Calculate the interest you get for 1 year then substract the the tax from it exmpl. if you have 100 dollar interest and you have 0.18 tax = 18% you will need to take away 18 dollar form 100.
Then you add this to the total amount of money he has.
When he reaches the desired sum you need to be able to tell how many years it took.

Hope this clears things up

this saved me :D thanks!