equity question (investment in WC)

Can someone please explain to me where the number 41 for Investment in WC comes from on question 13.A of Reading 41 (page 456 - Equity)? I would like to know also in general how to find Investment in working capital given a balance sheet. Thanks in advance.

You would have to take the current assets (except for cash) and subtract all current liabilities (except interest bearing debt) for 2 years. The change is your investment in working capital.

Thanks!