One person does a job in 4 hours and another in 7 hours. How long together?
Working together they take 28/11 hours (about 2.5454545 hours, or 2 h 33 min).
How to work it out
- 1
Think in rates: in one hour the first does 1/4 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 11/28 of the job per hour, the whole job takes the reciprocal:
- 4
Check: that is less than 4 hours, the faster worker alone, as it should be ✓
LaTeX source of the formulas on this page
\[ \frac{1}{4} + \frac{1}{7} = \frac{1}{t}\ \Rightarrow\ t = \frac{28}{11} \]
\[ \frac{1}{4} + \frac{1}{7} = \frac{7 + 4}{28} = \frac{11}{28} \]
\[ t = \frac{28}{11} = \frac{28}{11} \approx 2.5454545\text{ hours} \]Questions
How long do they take together?
Working together they take 28/11 hours (about 2.5454545 hours, or 2 h 33 min).