P can do a work in 10 days and Q can do the same work in 15 days. If they work on it together for 3 days, then the fraction of the work that is left is:

$\frac{1}{2}$

P = 10 days, Q = 15 days,

⇒ P + Q worked for 3 days = (3 + 2) x 3 = 5 x 3 = 15 units,  ..(Efficiency × Days = Total work)

⇒ Remaining work = 30 - 15 = 15 units,

⇒ Fraction of work left = \(\frac{15}{30}\) = \(\frac{1}{2}\)