The way this site works is that if you bid, you have to pay that amount, even if you lose. Bids are incremented by 1 cent. Let’s say an item sells for 10 cents. The guy who bid 1 cent still has to pay that, the guy who bid 2 cents still has to pay that, and so forth. So, what does the auction site actually earn for selling that item for 10 cents?

Notice that 1+2+3+4+5+6+7+8+9+10 can be added up by grouping numbers from the opposite ends: This is just or 11*5 = 55. Note that when x = 10, and we ended up multiplying for the series sum.

So, the general formula is:

1 + 2 + 3 + … + n =

Pop Quiz! If the sunglasses in the photo end up selling for $6.96, how much does the website make? = = = !!