Answer:
A Loss of $10,000
Explanation: 
To calculate the depreciation using the straight line method.
Depreciation = Cost - Salvage value/ no. of years
    
        $50,000   -   $10,000/ 4 = $10,000
Annual depreciation now is:    $10,000
Net book Value (NBV) for the year of disposal i.e 2018 will be:
Cost - Accumulated Depreciation = NBV
$50,000 - $30,000 = $20,000
NBV is $20,000
but was sold for $10,000 which is a loss of $10,000