In an AP the 3rd term is 16, and the 7th term is 12 more than the 5th term. Find the 20th term.

$a_7-a_5=2d=12$, so $d=6$. Then $a+2d=16$ gives $a=4$, and $a_{20}=4+19\times 6=4+114=118$. Treating the gap as 4$d$ would give $d=3$ and the wrong answer 67; using 20$d$ instead of 19$d$ gives 124 and using 18$d$ gives 112.