If we want to know how many widgets Bob assembles in 4 hours, we need to know his constant RATE of widget assembly.

1) Bob produces 20 widgets in a full workday.
We know that a full workday is 8 hours, so this is sufficient to tell us that his rate is 2.5 widgets per hour.

Or, you can infer that if he makes 20 widgets in a full day, he'll make 10 in half a day. Sufficient.

2) Bob produces 50 widgets in 20 hours.
If we have an output in a given amount of time, this is sufficient to tell us his rate - you don't even need to calculate. If you did, though:
50/20 = 5/2 = 2.5 widgets per hour. We can easily use this to calculate how many widgets in 4 hours: (2.5)(4) = 10.

Or again, you can divide: 20 hours is 5 times as long as 4 hours, so he'll produce 5 times as much. 50/5 = 10 in 4 hours.

The answer is D.