The average of D₁ and D₂ is 40. The average of D₂ and D₃ is 45. The average of D₁ and D₃ is 55. What is the value of D₃ ?

60

Sum of D₁  & D₂   =   2 × 40 = 80

Sum of D₂ & D₃   = 2 × 45 = 90

Sum  of D₁  & D₃   = 2  × 55 = 110

By observing all ,

 2(D₁  + D₂  + D₃)  = 110 + 90 + 80 = 280

D₁  + D₂  + D₃ = 140

80 + D₃  = 140

D₃  = 60

So , Value of D₃  is 60