Not to sound rude, since i am asking for help. However, I do not think that 60+ (n-1)*20 is equal to 40*20n.
60+(n-1)*20 does however equal 40+20n. Also, I do not understand how you got \sum_{n=1}^{X}(40+2n). Well really I don't understand where the (40+2n) comes from. Thank you for helping me...
Every time you buy a card the price goes up 20 units. The first card costs 60 units. How would you write an equation to find the TOTAL cost for X cards bought? I know how to get a price of an individual card by using the equation f(x)=20X+40. How would I write an equation that would add up the...