Let the total work be 90 units. Efficiency of X=90∕30=3 units/day Efficiency of Y=90∕45=2 units/day Efficiency of Z=90∕90=1 unit/day Let total time taken to complete the work be 2 d days Here X has worked for all the time but Y and Z has worked for only half the time. According to the question - ( Efficiency of X )×2d+( Efficiency of Y +Z)×d=90 ⇒3×2d+(2+1)d=90 ⇒9d=90 ⇒2d=20 days ∴ The work can be completed in 20 days.