Q. 16. nth term of a sequence is given by the formula an = 10-3n, find the sum of its 20 terms. Q. 17. The 8th term of an A.P. is 32 and its 12th term is 52. Find the A.P. Q. 18. 500 logs are stacked in a way such that 32 logs in the bottom row, 31 in the next row, 30 in the row next to it and so on. In how many rows are the 500 logs placed and how many logs are in the top row?