(i) The sum of an arithmetic progression is given by the formula:

\(S_n = \frac{n}{2} (2a + (n-1)d)\)

Given \(S_n = 525\), \(n = 25\), and \(a = -15\), we have:

\(525 = \frac{25}{2} (2(-15) + 24d)\)

\(525 = \frac{25}{2} (-30 + 24d)\)

\(525 = 25(-15 + 12d)\)

\(21 = -15 + 12d\)

\(36 = 12d\)

\(d = 3\)

(ii) The last term \(l\) in the progression is given by:

\(l = a + (n-1)d\)

\(l = -15 + 24 \times 3\)

\(l = -15 + 72\)

\(l = 57\)

(iii) The positive terms start from 3 and go up to 57. The first positive term is 3, which is the second term in the sequence:

\(a + d = 3\)

\(-15 + 3 = 3\)

The number of positive terms is 19 (from 3 to 57). The sum of these terms is:

\(S_{19} = \frac{19}{2} (3 + 57)\)

\(S_{19} = \frac{19}{2} \times 60\)

\(S_{19} = 570\)