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I was having problem answering this part of the question,

It say to do a journal entries for: Sales of Service 100 of which 20 was on credit.
My Answer and thoughts
I think it needs two separate journals entries but I am not sure. Here is what I come up: with

A) Title of account Deb Cre
Cash 80
Service Revenue 80

B) Account Receivable 20
Service Revenue 20

I am not sure of this work please help if you can and if you have another better way please post it

Actually you can combine it into one compound Journal Entry.

Debit Cash 80
Debit Accounts Reveivable 20
Credit Service Revenue 100