Answer:
[tex]\frac12p+2 = 3\frac12[/tex]
Step-by-step explanation:
The time he spends is the sum of the mandatory 2 hours and p times the half hour per puzzle. It has to end up to 3 1/2, so that's how you get the equation.
Of course, you can solve it as
[tex]p = \frac{1\frac12}{\frac12} = 3[/tex]
