AMO 2027 Grades 2-3 SET 10 - Self-study Question 13

Alice was travelling cross-country. The first day, she travelled 80 kilometres; the second day, 100 kilometres; the third day, 70 kilometres; the fourth day, 90 kilometres; and on the fifth day she travelled 60 kilometres. How many kilometres did she travel on the 15th day?

Self-study rule (editorial): continue the alternating changes of plus 20 kilometers, then minus 30 kilometers.