Basically, the key words "customers on account" means that you're going to be using Accounts Receivable at some point. Basically, you are receiving $900 from a debtor whom owes you money.
At some point in the past, you issued an invoice for $900. Let's say it was for a piece of equipment which they bought off you on credit. So the entry would have been:
Dr Accounts Receivable $900
Cr Equipment $900
(Ignoring any accumulated depreciation, revaluations, gain/loss on disposals, etc for anyone who wants to be smart
)
Now you would have issued an invoice for $900 to that person (lets say it just a customer in this case, though its the same entry with multiple customers kind of, but they haven't paid you cash yet. They owe you $900, so thats in your Accounts Receivable now, as an asset.
Now your entry is that, they have now paid you $900. So you would now transfer that $900 from Accounts Receivable into Cash at Bank, as you have received it now. So the entry would be:
Dr Cash at Bank $900
Cr Accounts Receivable $900
Hope that helps (and I hope that's what you were asking!). If you have to seperate multiple customers out, then really you'd just have a seperate Accounts Receivable account for each debtor and just divide up the $900 across them. Same entries as above.
All of this assumes your using a double entry, accrual based accounting system of course!