Helpp !! C2 question on geometric

HEY :P

Ginny opens a savings account and decides to pay £200 into the account at the start of
each month. At the end of each month, interest of 0.5% is paid into the account.
a Find, to the nearest penny, the interest paid into the account at the end of the
third month. (4)

amount in account after the 3rd payment in
=200+(1.005x200)+(1.005^2 x200)
=603.005
interest paid at end of 3rd month
=0.005x603.005=£3.02 (nearest penny)

i dont get why they did 603.005 x 0.005 ? could someone explain this to me .

thank you , would appreciate it