cash

Of course - you are calculating the cash received from receivables, and is some of the receivables turn out to be irrecoverable then you are not receiving the cash from these.

Bpp revision kit question 56 working capital finance.Why bad debt has not been considered in this example?

It is exactly as I wrote in my previous reply. The answer has only brought in 80 + 15 = 95% of the credit sales, because the other 5% is not received.