Pia and Ria can complete a project together in 26 days. Pia alone can complete the project in 78 days. They start working together but Pia leaves after 12 days. In how many days can Ria complete the remaining work of the project?

21 days

Pia + Riya = 26 days,

Pia = 78 days,

⇒ Pia + Ria = 3,   (Efficiency)

⇒ 1 + Ria = 3,

⇒ Ria = 2,

⇒ They both worked for 12 days and then Pia left = (1+2) x 12 = 3 x 12 = 36 units,  ..(Efficiency × Days = Total work)

⇒ Remaining work = 78 - 36 = 42 units,

⇒ Time required by Ria to complete the remaining work = \(\frac{42}{2}\) = 21 days,    ..(\(\frac{Work}{Efficiency}\) = Time)