15,21,18,19,21,17,24,15.这组数字的规律

题目

15,21,18,19,21,17,24,15.这组数字的规律
15,21,18,19,21,17,24,15.帮忙看看这组数字的规律,

答案

15+3=18
18+3=21
21+3=24
24+3=27
21-2=19
19-2=17
17-2=15
15-2=13