At a concession stand, five hot dog left parenthesis s right parenthesis and four hamburger left parenthesis s right parenthesis cost $12.00 ; four hot dog left parenthesis s right parenthesis and five hamburger left parenthesis s right parenthesis cost $12.75 . find the cost of one hot dog and the cost of one hamburger .

Hotdogs = $1.00
hamburgers = $1.75

5 hotdogs + 4 hamburgers = $12.00
4 hotdogs + 5 hamburgers = $12.75
hotdogs = x
hamburgers = y

5x + 4y = 12
4x + 5y = 12.75
subtract the second equation from the first:
(5x + 4y = 12) - (4x + 5y = 12.75) =
1x - 1y = -0.75
x + 0.75 = y ( this means one hotdog plus $0.75 equals one hamburger)
now substitute "y" for "x + 0.75" in either equation to find x ( I used the first equation)
5x + 4y = 12
5x + 4( x + 0.75) = 12
5x + 4x + 3 = 12
9x + 3 = 12
9x = 9
x = 1 ( one hotdog costs $1.00)
now substitute 1 for "x" in either equation to find y ( I used the first equation again)
5x + 4y = 12
5(1) + 4y = 12
5 + 4y = 12
4y = 7
y = 1.75 ( one hamburger costs $1.75)

checking answers with either equation ( input $1.00 for hotdogs and $1.75 for hamburgers):
equation 1:
5 hotdogs + 4 hamburgers = $12.00
5($1.00) + 4($1.75) = $12.00
$5.00 + $7.00 = $12.00
$12.00 = $12.00 ✔

equation 2:
4 hotdogs + 5 hamburgers = $12.75
4($1.00) + 5($1.75) = $12.75
$4.00 + $8.75 = $12.75
$12.75 = $12.75 ✔