Heidi Company is considering the acquisition of a machine that costs $420,000. The machine is expected to have a useful life of 6 years, a negligible residual value, an annual net cash inflow of $120,000, and annual operating income of $83,721. The estimated cash payback period for the machine is:__________.
a. 5.1 years
b. 5 years
c. 4 years
d. 3.5 years