Answer
The prices (in dollars) are: one gallon of milk $3.5$, one bottle of water $1.5$, and one snack-size bag $0.75$
Work Step by Step
Step 1. Assume the prices (in dollars) are: one gallon of milk $x$, one bottle of water $y$, and one snack-size bag $z$
Step 2. Based on the given conditions, we have
$\begin{cases} 2x+5y+6z=19\\y=2z\\x=y+2 \end{cases}$
Step 3. Adding the last two equations, we have $x=2z+2$
Step 4. Using the substitution of $x,y$ in the first equation, we have $2(2z+2)+5(2z)+6z=19$ which gives $20z=15$ and $z=0.75$
Step 5. Back substitute the know values to get $x=3.5$ and $y=1.5$
Step 6. We conclude the prices (in dollars) are: one gallon of milk $3.5$, one bottle of water $1.5$, and one snack-size bag $0.75$
