If 1/3x + 1/2y = 7 and x + y = 15, what is the value of y?

Clear fractions in the first equation by multiplying by 6: $2x + 3y = 42$. The second equation gives $x = 15 - y$. Substitute: $2(15 - y) + 3y = 42$, so $30 - 2y + 3y = 42$, which gives $30 + y = 42$ and $y = 12$. Then $x = 3$. Check: $\frac{3}{3} + \frac{12}{2} = 1 + 6 = 7$, and $3 + 12 = 15$. A common wrong answer is 3, which is the value of $x$. Another is 42, which comes from not isolating $y$ after clearing fractions. Report the variable the question asks about.