Concept:The series uses alternating addition and subtraction, where the positive differences increase by 2 and the negative differences decrease by 2.Explanation:Start with 13.Add 18 to get 31.Subtract 16 to get 15.Add 20 to get the missing term.Subtract 18 to get 17.So, the missing term is 15+20=35.Check: 13+18=31, 31−16=15, 15+20=35, 35−18=17.Answer:35