I want to check if my answers to the following questions are correct. :)

A partial adjusted trial balance of Safin Company at January 31, 2010

Dr. Supplies 900
Dr. Prepaid Insurance 2400
Cr. Salaries Payable 800
Cr. Unearned Revenue 750
Dr. Supplies Expense 950
Dr. Insurance Expense 400
Dr. Salaries Expense 1800
Cr. Service Revenue 2000

Questions:
1. If the amount in Supplies Expense is the Jan 31 adjusting entry, and $850 of supplies was purchased in January, what was the balance in supplies on January 1?
2. If the amount in Insurance Expense is the Jan 31 adjusting entry, and the original insurance premium was for one year, what was the total premium and when was the policy purchased?
3. If $2700 of salaries was paid in Jan, what was the balance Salaries Payable at December 31, 2009?
4. If $1600 was received for services performed in Jan, what was the balance in Unearned Revenue at December 31, 2009?

My answers:
1. Supplies, Beg. = Supplies, End. - Supplies Purchased + Supplies Expenses
Supplies, Beg. = $ 1200

2. Total Premium = Insurance Expense x 12 (12 months = 1 year)
Total Premium = $ 4800
Half of the premium was left, therefore the policy was purchased 6 months ago

3. Salaries Payable, Beg. = Salaries Payable, End. + Salaries Paid
Salaries Payable = $ 3500

4. Unearned Revenue, Beg. = Unearned Revenue, End.
Would the $1,600 received for services performed in January affect the Unearned Revenue in the first place?

Clarifications:
Should Salaries Expense and Service Revenue be included in the computation for nos. 3 & 4, respectively? Unlike in nos. 1 & 2, nothing was stated in nos. 3 & 4 that these expenses and revenue were the Jan 31 adjusting entries. Could you please help me out with this?

1. Your formula is correct, however the answer should be $1,000

2. This is correct.

3. You need to start with your beginning Salaries Payable plus the amount of salaries paid in Jan minus your Salaries Expense.

4. This is similar to number 3. You start with your ending Unearned Revenue amount plus the amount received for services performed in Jan minus your Service Revenue amount.

Why is there a need to include Salaries Expense and Service Revenue in the computation? Nothing was mentioned that these expenses and revenues were incurred/earned during the month and it could've been that some accounts/entries were reversed (reversing entries)?

Salaries Payable is used to record salaries expense incurred in one accounting period, but will be paid in another period.

Unearned Revenue is used when a company receives payment in one accounting period for a good or service, but the company will provide goods or services in another accounting period.

Salaries Expense and Salaries Payable are linked together, also Unearned Revenue accounts and Revenue accounts are linked together.

So this exercise is used so that you will understand how amounts flow through different accounts and how they flow through different accounting periods.

Thank you so much!