The $90 is the difference of inventory worth under LIFO and FIFO in the year of 20x5, therefore we need apply the income tax rate of 20% to the LIFO reserve for 20x5;
The $10 is the additional amount of inventory in 20x6 with FIFO less inventory with FIFO in 20x5; in other words, the FIFO inventory in 20x5 is (A+90) while in 20x6 is (A+100), therefore, only 10 (A+100-A-90) will be taxed at 30%.
Hope this help works. Thanks.

lifo转fifo会多交税收，就是lifo reserve乘税率，如果税率不一样，就要用各自对应的lifo reserve和变化值分别乘税率。

I read through your explanation but I still don't get it . To calculate taxes on 20X6 reserve is just multiplying the change in reserve by tax rate, which ends up with $3,  why have to add up $18, I don't get the mechanism by doing this.         

还是不明白。 为什么要加上18呢

可是为什么20X5  是 life reserve 乘上税率 而 20x6 却是 the change in lifo reserve 乘上税率 ，而且两者之和才是 20X6的 tax reserve