Answer:
$2,775
Step-by-step explanation:
The computation of the GDP of the economy is given below:
But before that the total value sold by three firms would be determined 
Total amount of goods sold by X: 
 = X sells to Y + X sells to Z 
 = $150 + $75 
 = $225 
 Total amount of goods sold by Y: 
 = Y sells to X + Y sells to Z 
 = $200 + $50 
 = $250 
 Total amount of goods sold by Z: 
 = Z sells to X + Z sells to Y 
 = $300 + $250 
 = $550 
 Now 
Amount of goods generated by X 
= units of output × cost per unit 
= 250 units × $4 
= $1,000 
 Value of goods generated by Y 
= units of output × cost per unit 
= 300 units × $6 
= $1,800 
 Value of goods generated by Z 
= units of output × cost per unit 
= 500 units × $2 
 = $1,000 
 Now GDP is 
= [Goods generated by X - Valued added by X] + [Goods generated by Y - Value added by Y] + [Goods generated by Z - Value added by Z] 
 = [$1,000 - $225] + [$1,800 - $250] + [$1,000 - $550] 
 = $775 + $1,550 + $450 
 = $2,775