There are 5 x 1/2 = 10 halves of burgers made from 5 whole burgers.
Era's first and second friends got 1 + 2 = <<1+2=3>>3 slices together.
Her third and fourth friends got 3 + 3 = <<3+3=6>>6 slices together.
Era's four friends got 3 + 6 = <<3+6=9>>9 slices.
So, Era is left with 10 - 9 = <<10-9=1>>1 slice.
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