Time & Work
Work from Days
If A can do a piece of work in n days, then A's 1 day's work = 1/n.
Days from Work
If A's 1 day's work = 1/n. Then A can finish the work in n days.
When man A is twice as good man B in work.
Ratio of work done by A and B = 2 : 1
Ratio of times taken by A & B to finish a work = 1 : 2

Time & Distance
Speed = Distance/Time
Time = Distance/Speed
Distance = Time x Speed
When man A travels certain distance in x km/hr and man B in y km/hr.
Then the average speed = [2xy/x+y] km/hr