Given: Ratio of efficiencies of 3 labours say A, B and C is 1 ∶ 2 ∶ 3. Efficiency of A and C is double and B's becomes half. A, B and C can complete the work in 30 days. Formula used: Total work = Number of person × Average efficiency × Number of days M1D1H1/W1 = M2D2H2/W2 (where M is number of men, D is number of days, H is number of hours and W is amount of work/wages.) Calculations: Let the efficiencies of A, B and C be 1unit/day, 2 units/day and 3 units/day respectively. Let the number of days required to complete the work after change in efficiencies be 'd days'. Let the total work be '1'. Initial average efficiency = (1 + 2 + 3)/3 ⇒ 2 units/day Final average efficiency = (2 + 1 + 6)/3 ⇒ 3 units/day According to question, ⇒ (3 × 2 × 30)/1 = (3 × 3 × d)/1 ⇒ d = 20 days ∴ The number of days required to complete the work after change in efficiencies is 20.