I am making this game, where there is system to upgrade buildings with money.
System is following, if you fund the building 100 percent, you get it done in 5 (game) weeks.
Lets suppose 100 percent funding is 100 USD.
However, if you want to speed up the building time, you can put additional money into it, but you can never reach more speed than 200 percent (and actually reaching that is impossible).
The system is following.
Suppose you need 100 USD to reach 100 percent.
You put another 100 USD more, and you can reach 150 percent.
If you after that put even another 100 USD (that is 300 USD in total), you can reach up to 175 percent.
next 100 USD again, will make it go to 187.5.
By other words, each 100 USD after the 100 percent, always results only in half the power before.
But how do I implement this mathematically. That suppose someone puts 135 USD of money in to building that building, how fast should it be building (percentage wise) then?