First I find the individual widget weight
81 - 73 = 2x
8 = 2x
x = 4
Then I find how many widgets were left
73 - 1 = 4y
72 = 4y
18 = y
Y is the amount of widgets left, therefore the amount left is 18
During 7 hours: 28 parts assembled
during 14 hours: 56 parts assembled
during 21 hours: 84 parts assembled
during 28 hours: 112 parts assembled
during 35 hours: 140 parts assembled