Please help me with this question: ( Value added method)

Calculate Value added by firms x and y

Sales by firm X -100

Sales by firm Y- 500

Purchases by households from firm Y- 300

Export by firm Y- 50

Change in stock of firm X- 20

Change in stock of firm Y- 10

Imports by firm X - 70

Sales by firm Z to Y -250

Purachases by firm Y from X - 200