Concept:An arithmetic progression's nth term is an=a+(n−1)d.Explanation:Given the progression −1,−3,−5,…, first term a=−1 and common difference d=−2.We need n such that an=−39.Using the formula: −39=−1+(n−1)(−2).Simplify: −39=−1−2(n−1).Add 1 to both sides: −38=−2(n−1).Divide by −2: 19=n−1.Thus, n=20.Answer:The 20th term is −39.