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问题: 15+16+17+18+19+20+21+22+23

15+16+17+18+19+20+21+22+23
=
=
=

解答:

15+16+17+18+19+20+21+22+23
=31+(17+23)+(18+22)+(19+21)+20
= 31+40+40+40+20
=31+140
=171