One person does a job in 3 hours and another in 3 hours. How long together?
Working together they take 3/2 hours (about 1.5 hours, or 1 h 30 min).
How to work it out
- 1
Think in rates: in one hour the first does 1/3 of the job and the second does 1/3.
- 2
Together in one hour they do the sum of the rates:
- 3
If they do 6/9 of the job per hour, the whole job takes the reciprocal:
- 4
Check: that is less than 3 hours, the faster worker alone, as it should be ✓
LaTeX source of the formulas on this page
\[ \frac{1}{3} + \frac{1}{3} = \frac{1}{t}\ \Rightarrow\ t = \frac{3}{2} \]
\[ \frac{1}{3} + \frac{1}{3} = \frac{3 + 3}{9} = \frac{6}{9} \]
\[ t = \frac{9}{6} = \frac{3}{2} \approx 1.5\text{ hours} \]Questions
How long do they take together?
Working together they take 3/2 hours (about 1.5 hours, or 1 h 30 min).