On January 1, 2016, Lefty, Inc. purchased a machine for $6,000. The estimated life and residual value were five years and $150, respectively. The machine will produce approximately 80,000 units over its life. In 2016 and 2017, the machine produced 10,000 and 15,000 units, respectively. In 2017, Lefty recorded $1,440 of depreciation expense. The depreciation method in use was the

straight-line method

sum-of-the-years'-digits method

double-declining-balance method

activity method