TD - Present value using annual, semi-annual or continuous discounting

QUESTION
Assuming an interest rate of r and discrete annual payments, what is the
present value of a sum of money to be recieved in T years’ time, i.e., how
much would you invest now to obtain the desired sum in T years? Hence
what is the present value of £100 to be recieved in 10 years time when the
interest rate is 5% and
(a) annual discounting is used;
(b) semi-annual discounting is used;
(c) continuous discounting.