Let p, q and 3 be respectively the first, third and fifth terms of an AP. Let d be the common difference. If the product (pq) is minimum, then what is the value of d?

Let S_k denote the sum of first k terms of an AP. What is $\rm \frac{S_{30}}{S_{20}-S_{10}}$ equal to?

If the harmonic mean of 60 and x is 48, then what is the value of x?

If the first term of an AP is 2 and the sum of the first five terms is equal to one-fourth of the sum of the next five terms, then what is the sum of the first ten terms?