profit on disposals

gary purchased a vehicle on 1 January 2006. The vehicle cost $20,000 and was depreciated on a straight-line basis over four years with no residual value. On 1 January 2009 Gary part-exchanged the vehicle for a new one costing $25,000. Gary paid cash of $18,000 towards the purchase of the vehicle. sir, the depreciation would be then $20,000 because 20000/4= 5000*4. is it correct? & in the disposals account, the profit would be $18,000 because, Cr Acc Depn, Dr original cost and i also Cr the part exchange which costs $25,000. I didn't include the cash paid ($18,000) for the new vehicle. is my answer correct for the profit in the disposal account?