#### Newbie: Need to average a textview

I will give an example of what I am trying to do with my code. I have 2 activities. On the first activity I have a price that's generated by a random number. User enters an amount to buy at that price. So for example: price is 1000 and they buy 2. I am sending that to activity 2. So now activity 2 should read 2 @ 1000. Now when new numbers get generated on activity 1 I need to average that in and send it to activity 2. So if the new random number is 500 and the user buys 2 of those I need to average it. Example:
2 @ 1000
2 @ 500
4 / 3000
average = 750

Display the average. When user sells any of those 4 I still need the same number to be displayed, unless the quantity reaches 0 then it resets.

Right now I have activity 2 displaying the total cash spent, but when I do the division it only divides the last amount(last total cash spent, and not the total cash spent all together) by the total quantity.

This is frustrating me lol. But I know I am doing something wrong. Here is what I have so far.

Variables: cost, quantity, totalCost, totalCash, avgCash;

totalCost = cost * quantity;
totalCash = totalCash + totalCost;

^ this sends the total cash to activity 2 just fine.
no matter how many they buy or the cost it still totals on
activity 2 just fine.

But when I add this line after it:
avgCash = totalCash / quantity;

It only divides the last totalCost by the total quantity, but yet what's stored in totalCash is the full amount. So why is it not dividing correctly?? I am a noob so be easy on me

I am new so I am not even sure I am doing this the right way or best way. I have not had a problem until now creating this app.
Any help would be appreciated, thanks.