One person does a job in 5 hours and another in 11 hours. How long together?
Working together they take 55/16 hours (about 3.4375 hours, or 3 h 26 min).
How to work it out
- 1
Think in rates: in one hour the first does 1/5 of the job and the second does 1/11.
- 2
Together in one hour they do the sum of the rates:
- 3
If they do 16/55 of the job per hour, the whole job takes the reciprocal:
- 4
Check: that is less than 5 hours, the faster worker alone, as it should be ✓
LaTeX source of the formulas on this page
\[ \frac{1}{5} + \frac{1}{11} = \frac{1}{t}\ \Rightarrow\ t = \frac{55}{16} \]
\[ \frac{1}{5} + \frac{1}{11} = \frac{11 + 5}{55} = \frac{16}{55} \]
\[ t = \frac{55}{16} = \frac{55}{16} \approx 3.4375\text{ hours} \]Questions
How long do they take together?
Working together they take 55/16 hours (about 3.4375 hours, or 3 h 26 min).