The hotel is $250 plus $25 per person so the equation would be
y= 25x + 250
(Y is the total cost and x is the number of people)
the restaurant is $100 plus $40 per person so the equation would be
y = 40x + 100
What we want to know is which x value would make the y values equal to each other. So you can just replace the "y" in one equation with the other equation:
25x + 250 = 40x + 100
and then solve...
1. subtract 100 from both sides
25x + 150 = 40x
2. subtract 25x from both sides
150 = 15 x
3. divide both sides by 15
10 = x
you can check by plugging 10 into the original equations
25(10) + 250
250 + 250 = 500
and
40(10) + 100
400 + 100 = 500
they both cost $500 when you invite 10 people
