Half the people at the party is 16 / 2 = <<16/2=8>>8 people.
8 people at the party eat 1 1/2 rolls each, 8 x 1 1/2 = 12 dinner rolls eaten.
8 people at the party eat 1/2 a roll each, 8 x 1/2 = <<8*1/2=4>>4 dinner rolls eaten.
Combined, the people at the party ate 12 + 4 = <<12+4=16>>16 dinner rolls.
There were originally 40 dinner rolls - 16 that got eaten = <<40-16=24>>24 dinner rolls leftover.
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