The cost of a party is $550. the price per person depends on how many people attend the party.

Y=px
y is the total cost, p is the number of people, x is the price per person
550=px

550=x(25)
You now know that 25x is equal to 550. But you need x, not twenty five x. To get rid of the 25, divide both sides by 25. 550/25=22 and 25x/25=x so x=22.
x=22

550=x(50)
x=11

550=(55)x
x=10

The answer is the third choice