Question

Working alone, Natalie can clean an attic in 9 hours. Lea can clean the same attic in 8 hours. How long would it take them if they worked together?

Work it out please!!!
A) 5.47 hours
B)3.78 hours
C)4.74 hours
D)4.24 hours

Answer 1

(1 attic / 8 hrs) + (1 attic / 9 hrs) = (1 attic / t hrs)

1/8 + 1/9 = 1/t
find common denominator
9/72 + 8/72 = 1/t
17/72 = 1/t
flip or cross multiply
72/17 = t
4.24 hrs = t