You can 
put this solution on YOUR website! .
From the problem, it is clear that they bought one pizza;
so they spent 15-8 = 7 dollars for toppings.
Thus you have this system of two equations
     x +  y = 5    (1)     (counting toppings)
    1x + 2y = 7    (2)     (counting money)
Easy check by inspection shows that only option B is the solution.
Solved.