An experienced car mechanic is working on changing the exhaust system. In 2010, the mechanic had performed this operation 100 times over her career. She estimates that, on average, it took her 80 minutes to change one exhaust system. By 2014, she had performed that operation 220 times, and it took her about 55 minutes to change one exhaust system. The learning curve in a log-log graph appears to be linear. By how much does the mechanic reduce the processing time of one operation with each doubling of the cumulative output?