Thanks Tetration your way will work well.
Here is another way :)
if $ 85 is put in account that gets 8.5%pa and I add $15 at the end of each year how much will I have at the end of 8 years
the 85$ will grow to 85*(1.085)^8
The first 15 will grow to 15(1.085)^7
The second 15 will grow to 15(1.085)^6
........
The last 15 only just goes in the bank so it will be 15
$$\\15+15(1.085)+15(1.085)^2 .........15(1.085)^7$$
This is a GP a=15 r=1.085 n=8
$$\\S_n=\frac{a(r^n-1)}{r-1}\\\\
S_8=\frac{15(1.085^8-1)}{1.085-1}\\\\
S_8=\frac{15(1.085^8-1)}{0.085}\\\\$$
so total = $$85(1.085)^8+\frac{15(1.085^8-1)}{0.085}\\\\$$
$${\mathtt{85}}{\mathtt{\,\times\,}}{\left({\mathtt{1.085}}\right)}^{{\mathtt{8}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{15}}{\mathtt{\,\times\,}}\left({{\mathtt{1.085}}}^{{\mathtt{8}}}{\mathtt{\,-\,}}{\mathtt{1}}\right)}{{\mathtt{0.085}}}} = {\mathtt{325.710\: \!957\: \!814\: \!451\: \!144\: \!3}}$$
After 8 years it will be $325.00