Jacob collects 800+1700= <<800+1700=2500>>2500 milliliters of water from the river and rain every day.
Since 1 liter is equal to 1000 milliliters, then 50 liters is equal to 50x1000= <<50*1000=50000>>50,000 milliliters of water.
Therefore Jacob needs 50,000/2500= <<50000/2500=20>>20 days to fill his water tank.
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