it takes karen 5 hours to stuff a case for envelopes. when she works with Anna, the job takes 3 hours. how long will it take Anna to stuff a case of envelopes by herself
Karen's rate = 1/5
Anna's rate = 1/a
combined rate = 1/5 + 1/a = (a + 5)/(5a)
1/[(a + 5)/(5a)] = 3
5a/(a+5) = 3
5a = 3a + 15
2a = 15
a = 7.5
Anna would take 7.5 hours