The proportion can be written
... days/(16 L) = (5)/(10 L)
Multiplying by 16 L gives
... days = (16 L)×5/(10 L) = 8
It will take Scott 8 days to jar 16 L of jam.
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Or, you can consider that Scott jars 2 L per day. (= (10 L)/(5 days)). At that rate, it will take him (16 L)/(2 L/day) = 8 days. This is the same as the above proportion written "upside down":
... (16 L)/(days) = (10 L)/(5 days)
Multiplying by the denominator on the left, and dividing by the ratio on the right, this becomes ...
... (16 L)/((10 L)/(5 days)) = days = (16 L)/(2 L/day) . . . . . . as above
You can solve the proportion from any of the 4 ways it can be written, but it can be easier to solve if the unknown is in a numerator. Another way to write it with the unknown in the numerator is ...
... days/5 = (16 L)/(10 L)
... days = 5×1.6 = 8 . . . . . . multiplying by 5