Treat the whole job as 1 unit. A person who finishes the job in n days does 1/n of it each day; rates of people working together add up.
1Rates
If A can do a work in n days, A's 1 day's work = 1/n
If A's 1 day's work = 1/n, A finishes the work in n days
A and B together: 1/a + 1/b per day, so they take ab/(a + b) days
2Efficiency
If A is k times as efficient as B, A takes 1/k of B's time
Ratio of work done = ratio of efficiencies (for equal time)
3Man-days
M₁ × D₁ × H₁ / W₁ = M₂ × D₂ × H₂ / W₂ (men, days, hours per day, work)