A dry goods company buys a new, modern mixer to supplement the output of their old, current mixer. The old mixer has a capacity of 50 gallons and can thoroughly mix a full load of loam and fill in 40 minutes. The new mixer has a capacity of 20 gallons and can thoroughly mix a full load of loam and fill in 15 minutes. Both mixers are loaded and started at the same time at the beginning of the day. Which function represents the output of the two mixers combined, where t represents the time in minutes since starting? Assume that emptying and reloading the mixers can be done instantly.