How long does it take for Sydney to clean the house?
1 / M + 1/ S = 1/3
1/5 + 1/S = 1/3 solve for S the time alone for Sydney to clean the house
Another way (when you have TWO people)
S * M / (S+M) = 3
5S = 3S + 15
2S = 15
s = 7.5 hr
+36915 
