Irrecoverable

Hi sir, I have confusion in calculation Case 1-Trade recievable totalled 864000 and allowance receivable was 48000, it was decided debts totalling 13000 and allowances adjusted to be 5%. Here need to calaculate allowance for receivable. So it is calaculated as Allaowance receivable=(864000-130000)x5% But in others other question it is calculated different. Case 2-Trade receivable was 863000 and it was written off balance totalling 23000.the allowance receivable adjusted to equilavent 5%. Here allaowance is calculated as 863000x5% Debts are not subtracted to find allowance. Case 3-Trade receivable 1232000 of 600000 which identified as unrecoverable and was written off ,allowance adjusted 5% of remaing trade receivables. Allowance receivable calculated as =(123200-60000)x5% My question is why in every question the allowance receivable is calculated different? Second question If we wanted to find out to i Total irrevoverable debts in question how help me.generally calculate,i watched videos but still not clear.