The nth term of an AP is $a_n=7n+4$. How many of its terms are less than 100?

$7n+4<100$ gives $7n<96$, so $n<13.71$ and $n$ can be 1, 2, ..., 13 - that is 13 terms$.(a_{13}=95<100$ while $a_{14}=102$.) Rounding 13.71 up to 14 counts a term that is already too big, and 95 is the value of the largest such term, not how many there are.