Given: 20 builders can excavate a building in 50 days.
Formula used: (M1 × D1)/W1 = (M2 × D2)/W2, where M = Number of workers, D = Number of days and W = work
Calculation: Let total work be 50unitsEfficiency of 20 builders = 50/50 = 1 units/day
After 10 days, 20 more builders join them, that is 40 builders work after 10 days
Work left after 10 days = 50 - 10 = 40 units
Let the required number of days be D
=(20+20)× ⇒
20×40=40×D ⇒
D=20 days
Alternate Method Let 1 builder do 1 unit/day work.
Hence, total work =
20×50×1=1000 units
Work done by 20 builders in 10 days = 20 × 10 = 200 units
Now, remaining work =
1000−200=800 units
∴ Required time =
==20 days.