Answer:
$40,000
Step-by-step explanation:
As we know that net working capital is 
Net working capital = Current assets - current liabilities
where, 
Current assets = Increase in inventory + increase in account receivable 
 = $30,000 + $41,000
 = $71,000
And, the current liabilities 
= Increase in account payable 
= $31,000
So, the effect on the firm net working capital is 
= $71,000 - $31,000
= $40,000