The manufacturer of brand A jogging shoes wants to determine how long the shoes last before resoling is necessary. She randomly samples from users in Chicago, New York, and Seattle.
In Chicago, the sample size was 28, and the mean duration before resoling was 7.2 months.
In New York, the sample size was 35, and the mean duration before resoling was 6.3 months.
In Seattle, the sample size was 22, and the mean duration before resoling was 8.5 months. What is the overall mean duration before resoling is necessary for brand A jogging shoes?