Sai can do a piece of work alone in 50 days. Ria is 25% more efficient than Sai. What is the number of days that Ria will take to do the same piece of work alone?

40 days

Sai = 50 days,

According to the question,

Ria is 25% more efficient than Sai, therefore,

⇒ Ria : Sai = 125 : 100 = 5 : 4  (Efficiency)

⇒ Total work = Sai x 50 days = 4 x 50 = 200 units,    ..(Efficiency × Days = Total work) 

⇒ Time required by Ria to complete total work alone = \(\frac{200}{5}\) = 40 days,   ..(\(\frac{Work}{Efficiency}\) = Time)