One person does a job in 2 hours and another in 9 hours. How long together?
Working together they take 18/11 hours (about 1.6363636 hours, or 1 h 38 min).
How to work it out
- 1
Think in rates: in one hour the first does 1/2 of the job and the second does 1/9.
- 2
Together in one hour they do the sum of the rates:
- 3
If they do 11/18 of the job per hour, the whole job takes the reciprocal:
- 4
Check: that is less than 2 hours, the faster worker alone, as it should be ✓
LaTeX source of the formulas on this page
\[ \frac{1}{2} + \frac{1}{9} = \frac{1}{t}\ \Rightarrow\ t = \frac{18}{11} \]
\[ \frac{1}{2} + \frac{1}{9} = \frac{9 + 2}{18} = \frac{11}{18} \]
\[ t = \frac{18}{11} = \frac{18}{11} \approx 1.6363636\text{ hours} \]Questions
How long do they take together?
Working together they take 18/11 hours (about 1.6363636 hours, or 1 h 38 min).