Question 225621
the combined time is less than either individual time


the FRACTION filled by an individual is "combined time"/"individual time"


the FRACTIONS sum to 1, the WHOLE task


let x="combined time"


x/15 + x/12 = 1


multiplying by 60 (LCD) ___ 4x + 5x = 60 ___ 9x = 60


dividing by 60 ___ x = 20/3 = 6hr 40min