One person does a job in 2 hours and another in 7 hours. How long together?
Working together they take 14/9 hours (about 1.5555556 hours, or 1 h 33 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 1/7.
- 2
Together in one hour they do the sum of the rates:
- 3
If they do 9/14 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
\[ \frac{1}{2} + \frac{1}{7} = \frac{1}{t}\ \Rightarrow\ t = \frac{14}{9} \]
\[ \frac{1}{2} + \frac{1}{7} = \frac{7 + 2}{14} = \frac{9}{14} \]
\[ t = \frac{14}{9} = \frac{14}{9} \approx 1.5555556\text{ hours} \]Questions
How long do they take together?
Working together they take 14/9 hours (about 1.5555556 hours, or 1 h 33 min).