Help with Control account reconciliations, Unit 5

Can someone please help!!!. I was merrily making my way though the BPP text book Chapter 10 on Control account reconciliations, all making perfect sense until they start talking about SCLA with both a debit and credit opening/closing balances, no explanation as to how or why. The example that is giving is making no sense to me at all

Debit side
Balance b/d 12,223
SDB 90

balance c/d(known) 645
balance b/d 12,133

Credit Side
Balance b/d 645
CRB 100
bad debt 80

Balance c/d (bal fig) 12,133
balance b/d 645

I don't understand how the new opening balances were arrived at. I am sure it is really simple, and I am being terribly thick. Please please someone take pity on me
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