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(1)120÷4+1=31(棵)
(2)120÷4=30(棵)
(3)120÷4-1=29(棵)
(4)两端种:(120÷4+1)×2=62(棵)一端种:120÷4×2=60(棵)两端都不种:(120÷4-1)×2=58(棵)