Hey I need a solution for part (b) only:

A bank has an account for investors. Interest is added to the account at the end of each year at a fixed rate of 5% of the amount in the account at the beginning of that year. A man and a woman both invest money.

(b) The woman decides that, to assist her in her everyday expenses, she will withdraw interest as soon as it has been added. She invests $y at the beginning ofeachyear. Show that, at the end of n years, she will have received a total of $[n(n+1)y]/40 in interest.

Thanks if you could help, I'm having my math paper in about 8 hours' time!