1) amounts due from customers at year-end were $28,000. Of this amount, $3,000 will not be collected.
--> I have DR. BDE 3,000.. . CR. ADA 3,000

2) Co. rents its building for $3,000 a month, payable quarterly in advance.
-->I have Dr. Rent Expense 9,000.. . CR. Rent Payable 9,000
Given Rent Expense of $45,000 on IS

3) equipment of $30,000 was purchased on January 1, 2005. The expected life is 5 years, no salvage value. (straight line depreciation)
Given Equipment Expense of $30,000 on Income Statement (Dec. 2005.)

There's some missing information.

1) What they are giving you is the analysis of receivables or aging of receivables method of estimating a dollar amount. (Versus the percent of sales method.) In this method, the amount they say is uncollectible is the balance you want to have in the uncollectible account, and you have to figure out what amount to put in your entry to make that balance happen. Meaning, you need to know the balance in the allowance account before your entry.
I have some examples of this in another post, which I will try to find and link to.

2) I can't tell you about the dollar amount. If it's paid quarterly in advance, then you're paying $9000 every quarter in advance. But you've given no information about what date it is, whether you're doing adjusting entries monthly or annually, when it was last paid, etc. So it's impossible to determine the dollar amount.

Normally the balance in the rent expense account would be irrelevant to the entry, and the balance in the prepaid rent account would be relevant, which you didn't include. However, there is an alternative way to do it, and giving the expense account instead of the prepaid account makes me wonder if they're using the alternative method. No way for us to know unless we know what account the original payment went into. It could be stated in the problem, it could be all the examples just do it an alternative way and we'd have to see an example, or if you have no balance in a prepaid account.

The only thing I can say is that it absolutely doesn't involve a payable of any kind. If you are paying it in advance, how can you still owe it? Payables are for things you still owe, not for things you've already paid.

3) Straight-line depreciation is:


That's the depreciation expense for each year. The entry for depreciation is always dr to Depreciation Expense and cr to Accumulated Depreciation.