A specialized engineering firm produces custom water-filtration systems. The table below shows the units of output, average total cost (ATC), and average revenue (AR) for the firm.
| Units of output | Average total cost (£) | Average revenue (£) |
|---|---|---|
| 50 | 120 | 180 |
| 60 | 115 | 170 |
| 70 | 112 | 161 |
| 80 | 111 | 153 |
At which of the following levels of output will the firm maximise its total profit?
50 units
60 units
70 units
80 units
