Hmm..
I would have approached it like this…
We know that LIFO COGS for a given periods is equal to LIFO COGS less the change in the reserve. So here, the reserve increased by 10,000.. so COGS under LIFO would be 10,000 less if you had used FIFO.
This would mean that you would have had $6000 more in net income for the period if you had used FIFO rather than lifo, since $10,000 *(1-t) = $6,000
Now that just reflects the impact of one period. By 2006, you have a lifo reserve that has grown to $60,000. If you reserve that entire bad boy, then you subtract $60,000 from LIFO cogs. $60,000 * (1-t) = $36,000… the benefit to net incomes (which flows out to retained earnings completely if the firm is not paying a dividend).