If three-quarters of the boxes are full, then 1 – ¾ = ¼ of the boxes are half-full.
On average, each harvest therefore has 20 boxes * 0.25 = <<20*0.25=5>>5 boxes that are half-full.
This leaves 20 total boxes – 5 half-full boxes = <<20-5=15>>15 full boxes.
Half-full boxes hold 20 parsnips / 2 = <<20/2=10>>10 parsnips each.
In total, the half-full boxes, therefore, hold 5 boxes * 10 parsnips = <<5*10=50>>50 parsnips.
The full boxes hold a total of 15 boxes * 20 parsnips = <<15*20=300>>300 parsnips.
So Betty harvests a total of 50 + 300 = <<50+300=350>>350 parsnips in an average harvest.
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