If t1 = 100, that means after zero washings. Some people might write "t0" for that, but that's a matter of convention. If t1 = 100, then t2 = 95 (after one washing), and t11 = 59.87, after 10 washings. You need to plug 11 in for n, not ten. Just a matter of how the counting is done.
Part e requires logarithms. If you're doing this, I suppose you're familiar with logarithms. Go back and review them. If you have problems, ask further questions.