First, the number of segments in the 2 millipedes that are twice as long is 60 segments * 2 = <<60*2=120>>120 segments.
Then one can multiply the segments by the 2 millipedes to find the total number of segments: 120 segments/millipede * 2 millipedes = <<120*2=240>>240 segments.
In total, we then find the scorpion has eaten one 60-segment millipede in addition to the 3 millipedes above for a total of 60 + 240 = <<60+240=300>>300 segments.
So, the number of segments it still needs to eat are : 800 segments - 300 segments = <<800-300=500>>500 segments
Thus, it needs to eat an additional 500 segments / 50 segments/millipede = <<500/50=10>>10 millipedes.
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