Wa-Alaykum As Salaam
Bismillah ArRahman ArRaheem
1. Once khums is paid it has been spent so there is no provision to count back the overpayment. However you may be able to seek permission from the
your marja otherwise.
2. In year 2: $5k khumsed money.
Year 3: loss of 1k: $4k Khumsed money.
Year 4: loss of 2k: $2k khumsed money.
Year 5: $10k savings. $2k is khumsed. Balance of money that is not
khumsed is $8k.
So the correct answer is b. $10k-$2k=$8K. Khums amount = $1.60K (20% of 8k)
May Allah (swt) bless your wealth.