The first term of an arithmetic sequence is 30 and the common differenc eis -1.5.

a) Find the value of the 25th term.

The rth term of the sequence is 0.

b)Find the value of r.

The sum of the first n terms of the sequence is Sn.

c) Find the largest postitive value of Sn.

i worked out a and b but got stuck with c. Any help!