On September 19, 2012, McCoy Co. purchased machinery for $285,000. Salvage value was estimated to be $15,000. The machinery will be depreciated over eight years using the sum-of-the-years'-digits method. If depreciation is computed on the basis of the nearest full month, McCoy should record depreciation expense for 2013 on this machinery of:

This is the solution: ($270,000 8/36 9/12) + ($270,000 7/36 3/12) = $58,125. What I need help with is figuring out how they got 9/12 and 3/12 thank you!