Correction of errors

Sir i have a query..
A credit note for $95 sent to Bury co had been correctly entered in the day book but was entered in the account in the receivable ledger as $75.
what is the revised balance of the receivable ledger after correcting these errors..
i understand the previous erros whether we have to subtract or add..but in this we are subtracting the credit note error? sir do we never add any credit note amount in the receivable ledger?
we are here doing this 49449-20=..why not adding 20? to make out the difference