Guys pls help

A contractor has agreed to build a storage depot for a target price of $90,000, but there is some uncertainty involved in the project, so buyer & seller have agreed that the ceiling price will be $100,000. The buyer will pay 100% costs  upto $80, 000 and 75% of costs between $80,000 & $100,000 but any cost over $100,000 is absorbed by the contractor. What is the point of total assumption for this contract for the buyer?

1. $100, 000

2. $90, 000

3. $87, 750.25

4. $93, 333.33

Ans: option 4.. how?