Presented below is a partial trial balance for the Kansas Instruments Corporation at December 31, 2011.

Account Title Debits Credits
Cash 22,000
Accounts receivable 141,000
Raw materials 33,000
Note receivable 114,000
Interest receivable 3,400
Interest payable 4,000
Marketable securities 35,000
Land 49,000
Buildings 1,350,000
Accumulated depreciation—buildings 670,000
Work in process 42,000
Finished goods 91,000
Equipment 360,000
Accumulated depreciation—equipment 137,000
Patent (net of amortization) 126,000
Prepaid rent (for the next two years) 64,000
Unearned revenue 36,000
Accounts payable 174,000
Note payable 410,000
Cash restricted for payment of note payable 71,000
Allowance for uncollectible accounts 11,900
Sales revenue 780,000
Cost of goods sold 460,000
Rent expense 26,000

Additional information:
1.

The note receivable, along with any accrued interest, is due on November 22, 2012.
2.

The note payable is due in 2015. Interest is payable annually.
3.

The marketable securities consist of treasury bills, all of which mature in the next year.
4.

Unearned revenue will be earned equally over the next two years.

Required:

Determine the company's working capital at December 31, 2011. (Omit the "$" sign in your response.)

This is my problem,

I have
Current Asset
Cash 22,000
A/R 141,000 - 11,900 ( allowance )
Note Receivable 114,000
Interest Receivable 3,400
Prepaid rent 64,000
Marketable securities 35,000
Raw materials + WIP + finish goods = 33k+42k+91K = 166k

Current liabilites:
Current maturity of long term liability 71,000
Interest payable 4,000
Unearn revenue 36,000
And Account payable 174,000

so I have 533,500-285,000 = 248,500 But somehow it is not correct. Please help me! What did I do wrong?

You are pretty close. The issues are:

Prepaid rent is for 2 years.
Unearned revenue is for 2 years.

Current amounts should only be those receivable or due within one year.

You aren't understanding the definition of cash restricted. This is actually cash put aside for a specific use, in this case towards paying off the long term debt. As cash, it is an asset - whether it is current or long-term depends on its intended use. If it's set aside for a long-term obligation, it is not current.