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?

Let's assume the total work is 78 units.

Pia and Ria can complete a project together in 26 days.

Efficiency of Pia and Ria together = $$\frac{78}{26}$$ = 3 units/day

Pia alone can complete the project in 78 days.

Efficiency of Pia = $$\frac{78}{78}$$ = 1 unit/day

Efficiency of Ria = 3-1 = 2 units/day

They start working together but Pia leaves after 12 days.

Let's assume the number of days taken by Ria to complete the remaining work of the project is 'y'.

12 $$\times$$ Efficiency of Pia and Ria together + y $$\times$$ Efficiency of Ria = total work

$$12\times3+y\times2 = 78$$

36+2y = 78

2y = 78-36

2y = 42

y = 21

So Ria can complete the remaining work of the project in 21 days.