At a party, everyone shook hands with everybody else. There were 66 handshakes. How many people were at the party?

Step-by-step explanation:

Other answers have already shown that there are  n=12  people at the party, either using combinatorial choose notation

(n2)=n!2!(n−2)!=66  

or by counting handshakes person by person, noting that each successive person has one fewer unique handshake than the prior (as the previously counted people are disregarded), and then using the sum-of-consecutive-integers formula to get

(n−1)+(n−2)+...+2+1=∑n−1k=1k=(n−1)n2=66  

and solving for n .

Here I will provide another method of arriving at n(n−1)2=66.

Instead of keeping track of unique handshakes from the start, note that each person shakes hands with  n−1  other people. As there are n  people at the party, this gives us n(n−1)  handshakes. However, notice that we counted each handshake twice; for example, if Alice and Bob shook hands, we counted that once for Alice, and then again for Bob. To account for the double-counting, we can divide by  2, which gives us the previously mentioned formula of n(n−1)2.

As a side note, if we did not previously know the sum formula, we could use this as a combinatorial proof to show that it works. Also, if we account for repeated counting by dividing by k! , the number of permutations of k  objects, a similar method can be used to derive the chosen formula  (nk)=n!k!(n−k)!.

6,250

step-by-step explanation:

to get the area, multiply 360 by 160.(57,600)

to find out how many bags of fertilizer it will take, first figure out how many square feet one bag can hold. divide 1000 by 8(125) to get the unit rate(how many square feet one pound of fertilizer can hold)and multiply it by 50(6250). if i haven't made a mistake, that is the answer.

imma say 16%. sorry if its wrong. i'm doing the same stuff right now

step-by-step explanation: