two hot dogs and two sodas--------------------$8.00
2x+2y=8------x+y=4 x=4-y
three hot dogs and one sodas--------------------$9.00
3x+y=9----------3*(4-y)+y=9
12-3y+y=9
3=2y------------------y=1.5
x= 4-1.5=2.5
Cost
1 hot dogs----------x=$2.5
1 soda--y-----------y=$1.5