Chapter 9 - Sequences and Series - 9-2 Arithmetic Sequences - Practice and Problem-Solving Exercises - Page 576: 65

The other term is $54$.

Recall: Arithmetic mean (or average) of two numbers $a$ and $b$ is $\dfrac{a+b}{2}$. Let $a$ = 30 $b$ = the other term Using the formula above gives: \begin{align*} \text{arithmetic mean} &= \dfrac{a+b}{2}\\\\ 42&=\dfrac{30+b}{2}\\\\ 2(42)&=2\left(\dfrac{30+b}{2}\right)\\\\ 84&=30+b\\\\ 84-30&=b\\\\ 54&=b \end{align*}