Need Help Finding Answer

[EneergE]

I am clueless how to approach this problem, because I have no formula what so ever and am trying to figure out how to do this because of my own curiosity. But here's the question:

If you have a cost of $40,000,000, that increases by $10,000 every time five units are bought, how can you determine how many units you can buy from a certain money amount? You can only buy 5 units at a time and the starting $40,000,000 is the cost of 5 units. <--- Finish this before doing the next question

Further complicating things, if you currently have $0, but you are earning $50,000,000 (but this number increases by $4,000) every 5 minutes, how can you determine how many units you can purchase from the above problem, after an hour. In understandable language: If your making $50,000,000/hour, but you receive a raise every 5 minutes of $4,000, how many units can you purchase from the first problem?

These are only part of the problem I'm trying to figure out. I think I can get the rest of it if I can just figure out how to find the solution to these two problems.

If someone can just point me in the right direction on how to determine all of this, that would be great. However, if you can't figure out the answer, I just need some kind of formula or a description of what kind of math that I'm trying to do here.

Thanks,

~ EneergE

[OfficeShredder]

Since units are sold in bricks of five, call each brick a single "unit" because I'm lazy at typing

The first unit costs 40,000,000. The second 40,000,000 + 10,000. Then 40,000,000 + 20,000. In general, unit n (the first being the 0th unit) will cost 40,000,000 + 10,000*n. So the cost of N units is the sum from 1 to N of 40,000,000*k + 10,000*(k-1). You can separate that into two sums, which you should be able to handle (they're arithmetic series)