Answer:
a) $543.75
b) $1,903.20 
c) $1,403.2 (loss)
Explanation:
Data provided in the question:
Cost basis for the device = $20,060
Useful life = 7 years
Salvage value = $0
Now,
Using the 200% declining balance method,
Rate of depreciation =  
or
Rate of depreciation =  
or
Rate of depreciation = 0.2857 per year or 28.57% per year
Now,
Book value at the end of first year
=  $20,060 - 0.2857 × $20,060
= $14,328.57
Book value at the end of second year
=  $14,328.57 - 0.2857 × $4093.67 
= $10234.9
Book value at the end of third year
=  $10234.9 - 0.2857 × $10234.9
= $7310.78 
Book value at the end of fourth year
=  $7310.78 - 0.2857 × $7310.78 
= $5222.08 
Book value at the end of fifth year
=  $5222.08 - 0.2857 × $5222.08 
= $3730.13
Book value at the end of fifth year
=  $3730.13 - 0.2857 × $3730.13
= $2664.43 
Book value at the end of sixth year
=  $2664.43 - 0.2857 × $2664.43 
= $1903.20 
Therefore,
a) The depreciation charge in the year 7 will be 
= 0.2857 × $1903.20 
= $543.75
b) Book value at the end of year six = $1,903.20 
c) The loss on the disposal of the device after six year for $500
= Selling cost - Book value 
= $500 - $1,903.20
= -$1,403.2          [Here, negative sign means loss]