An AP has common difference 4, and its first 12 terms add up to 372. Find the first term.

372 = $\frac{12}{2}$$[2a+11(4)]=6[2a+44]$, so $62=2a+44$, giving $2a=18$ and $a=9$. The trap 7 comes from using $12d=48$ in the bracket instead of $11d=44$. 31 is the average term $\frac{372}{12}$, which sits in the middle of the AP, not at its start. 5 fails the check, since $S_{12}$ would then be 348.