It&#x27;s a really standard form of 2 formula substitution, you&#x27;re missing the fact that there was 209 drinks and salads sold combined which constrains the solution to a single answer. From the setup you get 2 equations where x is the number of salads and y is the number of drinks:<p>&gt; [1] 6.5<i>x + 2 </i> y = 836.5<p>and<p>&gt; [2] x + y = 209<p>Solve [2] for y and substitute into [1]:<p>&gt; 6.5*x + 2(209 - x) = 836.5<p>You can also do this with linear algebra but this is the way I would have known as a normal high schooler taking the SAT.