21 labourers can build a wall in 25 days. If 14 additional labourers are employed, then in how many days the work can be completed ?
Answer: D. 15 days
Original labourers = 21 Additional labourers = 14 Total labourers = 21 + 14 = 35. 21 labourers can build a wall in = 25 days. 1 labourers can build a wall in = 25 x 21 days. 35 labourers can build a wall in = ${25 × 21}/35$ = 15 days