S alone can do a piece of work in 14 days. T alone can do the same work in 21 days. U alone can do the same work in 28 days. Working together, in how many days will they complete the work?

$\frac{84}{13}$ days

S = 14 days,

T = 21 days,

U = 28 days,

⇒ Time required by S + T + U to complete the work = \(\frac{84}{13}\) days.   ..(\(\frac{Work}{Efficiency}\) = Time)