Explanation & answer:
Cash basis, so all monies retain same values over the years.
Let x = payback period in years
Salvage value of machine
= 48000 - 4000x
Sales
= 16000x
Total revenue after x years
R = 16000x
Expenditures over x years
C = Cost of machine + materials + depreciation
= 48000 + 8000x + 4000x
= 48000 + 12000x
For payback
R = C
16000x = 48000 +12000x
Solve for x
x = 48000/4000 = 12 years
By that time, the machine has no more salvage value.