Answer: at least, 2931 board feet
Step-by-step explanation:
Let x represent the number of board-feet of lumber yards that is produced and sold in a day
The lumber yard has fixed costs of $3810.30 per day and variable costs of $0.91 per board-foot produced. It means that the cost of producing x board-feet of lumber yards is
0.91x + 3810.3
Lumber sells for $2.21 per board-foot. It meas that the revenue from selling x board feet of lumber yard is 2.21x.
Profit = Revenue - total cost
In order to make profit, Revenue must be greater than total cost. Therefore,
2.21x ≥ 0.91x + 3810.3
2.21x - 0.91x ≥ 3810.3
1.3x ≥ 3810.3
x ≥ 3810.3/1.3
x ≥ 2931
The number of board-feet of lumber yard that must be sold to make profit is at least 2931