a Machine A costs \(\$ 10,000\), has accumulated depreciation of \(\$ 4,000\) as of year-end, and is being depreciated on a straight-line basis over 10 years with an estimated salvage value of zero. How old is machine \(\mathrm{A}\) as of year-end?

b Machine B has accumulated depreciation (straight-line basis) of \(\$ 6,000\) at year-end. The depreciation charge for the year is \(\$ 2,000\). The estimated salvage value of the machine at the end of its useful life is zero. How old is machine B as of year-end?