Question 503463
Let 
Hotdog = h
Soft Drinks = s
10h+5s=12.5 ...........(1)
7h+4s=9................(2)
Use substitution method to solve above problem
Take {2)
7h+4s=9
7h=9-4s
Divide by 7 both sides of the equation
7h/7=(9-4s)/7
h=(9-4s)/7
Put the value of h in (1)
10h+5s=12.5 ...........(1)
10((9-4s)/7)+5s=12.5
((90-40s)/7)+5s=12.5
Multiply by 7 both sides of the equation
7*[((90-40s)/7)+5s]=12.5*7
90-40s+35s=87.5
-5s=87.5-90
-5s=-2.5
Divide by -5 both sides of the equation
-5s/-5=-2.5/-5
s=0.5
Put the value of s in any of two equations (1) or (2)
7h+4s=9................(2)
7h+4(0.5)=9
7h+2=9
7h=9-2
7h=7
Divide by 7 both side of above equation
7h/7=7/7
h=1

Hotdog costs = $1
Soft Drink cost =$0.5

Check
Put values of x and y in any of two equations (1) or (2)
10h+5s=12.5 ...........(1)
10(1)+5(0.5)=12.5
10+2.5=12.5
12.5=12.5

7h+4s=9................(2)
7(1)+4(0.5)=9
7+2=9
9=9