Sarthak can fill a sand-pit with sand in 36 days while Vivan takes 90 days to fill it. Ali can take the entire sand of a filled sand-pit out in 60 days. If all three start working when the pit is empty, in how many days will the sand-pit be full again?
we can solve this by Time and Work
here we substract the work of ali because he taken out the sand,
now time taken by all to fill $$\ \frac{\ total\ work}{work\ done\ by\ all\ in\ a\ day}=\ \frac{\ 180}{4}$$=45 days
hence option 'A' is correct
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