Sources of cash
A source of cash is one that increases cash flow in the period. For instance, selling a securities generates cash and so is a "source of cash." Selling a building for cash would also be a source of cash. So, selling assets for cash are always a "source of cash."
In an indirect cash flow statement, if accounts receivable is lower at the end of the period than at the beginning of the period, it ...
Your tutorial is 285 words and indicates the overall rule and give specific examples for assets and liabilities.