A small call center normally has five employees answering calls while open. On average, five calls arrive every four minutes. Under normal operating conditions, each employee on average handles each call in 3.5 minutes. But today, one employee has the flu, so the center needs to operate with four employees during this time. The manager is nervous that it may be supply-constrained and so she directs her employees to hurry up their processing of calls today. What is the maximum average processing time (in minutes) for each call that they need to achieve today so that they have a stable queue?