Final answer:
Mary and her roommate can clean the apartment together in approximately 2.73 hours. This is calculated by adding their individual cleaning rates and then taking the reciprocal of the sum to find the total time.
Step-by-step explanation:
The question asks how long it would take for Mary and her roommate to clean the apartment if they worked together. Mary can clean the apartment in 6 hours, which means in one hour, she can clean 1/6 of the apartment. Her roommate can clean the apartment in 5 hours, so in one hour, the roommate can clean 1/5 of the apartment. Working together, their combined cleaning rate per hour is 1/6 + 1/5. To find their combined rate, we get a common denominator, which is 30, so their rates become 5/30 and 6/30, respectively. Summing these, we get a combined rate of 11/30 of the apartment cleaned per hour. To find the amount of time it takes for them to clean the entire apartment together, we divide 1 by their combined rate, so 1 / (11/30) = 30/11 hours which is approximately 2.73 hours.