A cellphone repair shop consists of three processes. Step 1 requires 4 minutes per unit, step 2 requires 6 minutes per unit, and step 3 requires 6 minutes per unit. 20% of units that complete the third step require rework, which means those units must start the process over at step 2 (processing times are the same for units being reworked) and rework is always successful. Demand at the shop is 0.6 units per minute. What is the capacity of the shop (in units per minute)?