The rungs of a ladder are 20 cm apart. Their lengths fall evenly from 50 cm at the bottom to 30 cm…

In a word problem, find the quantity that changes by a fixed amount: that is the common difference $d$. Read $a$ and $n$ with their units. A single amount is a term, $a_n = a + (n - 1)d$; a total is a sum, $S_n = \dfrac{n}{2}[2a + (n - 1)d]$.