Time, work, speed & distance

Work together

Time = ab / (a + b)

A alone a days, B alone b days

Man-days

M₁D₁H₁ / W₁ = M₂D₂H₂ / W₂

Pipes

Net per hour = 1/a − 1/b

Filling minus emptying

Speed

Speed = Distance / Time

km/h × 5/18 = m/s

Train crossing a pole

Time = Train length / Speed

Train crossing a platform

Time = (L₁ + L₂) / Speed

Relative speed

Same direction: a − b; opposite: a + b

Boats & streams

Down = b + s, Up = b − s, b = (D + U)/2, s = (D − U)/2