Question help

Conditions of HDB flats would deteriorate over time and owners often undertake major renovations after staying in the same flat for 20-25 years

Mr. Goh hatched a plan to avoid such major renovations. His plan involves buying a resale flat with 95 years lease remaining and selling the flat off after staying in it for 5 years (the flat would have 90 years of lease remaining).

He would then repeat the process, that is, buy another resale flat with 95 years of lease remaining and selling it off after staying in it for 5 years. Mr Goh, figured that with his plan he will always get to stay in a "newish" resale flat with no worries about any major renovations.

Flats with 95 years of lease remaining would naturally cost more than flats with 90 years if lease remaining, so wach time Mr Goh repeats the cycle of selling and buying, he would incur a cost.

Let "K" be the resale price of a HDB flat with 95 years of lease remaining.
Let "S" be the resale price of a HDB flat with 90 years of lease remaining.

Assume the following:
1. Resale proces go up by 5% every 5 years.
2. Mr. Goh always buys same sized resale flats (flat-type, floor area).
3. There are no other costs involved in each transaction.

Write down using sigma notation, the expression that you can use to determine the total cost of Mr. Goh's plan if he were to buy his first flat at age 35 and his last flat at age 65.