Cost of posting 3 small, 9 medium and 2 large parcels = $94.45
Let the cost of a small parcel be x
Let the cost of a medium parcel be y
Let the cost of a large parcel be z
Cost of 6 small, 8 medium and 9 large parcels:
6x + 8y + 9z = 173.20------(1)
Cost of 7 small, 13 medium and 17 large parcels:
7x + 13y + 17z = 291.5-------(2)
Also, a large parcel costs twice as much as a small parcel, therefore
z = 2x
Substituting z = 2x in equation (1) and (2) we get:
24x + 8y = 173.20------(3)
41x + 13y = 291.5--------(4)
Multiplying equation (3) with 13 and equation (4) with 8 we get
312x + 104y = 2251.6
328x + 104y = 2332
By solving the equations we get
16x = 80.4
x = 5.02
Putting x = 5.02 in equation (3) we get:
y = 6.59
z = 10.04
Cost of posting 3 small, 9 medium and 2 large parcels: 3*5.02 + 9*6.59 + 2*10.04 = 15.06 + 59.31 + 20.08 = $94.45
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