If $a+b=7$ and $a^3+b^3=42$, what is the value of the sum $\dfrac{1}{a}+\dfrac{1}{b}$?  Express your answer as a common fraction.
Cube both sides of $a+b=7$ to find \[
a^3+3a^2b+3ab^2+b^3=343.
\] Substitute 42 for $a^3+b^3$ and factor $3ab$ out of the remaining two terms. \begin{align*}
42+3ab(a+b)&=343 \implies \\
3ab(a+b)&=301 \implies \\
3ab(7)&=301 \implies \\
3ab&=43 \implies \\
ab&=\frac{43}{3}.
\end{align*} Finally, $\frac{1}{a}+\frac{1}{b}=\frac{a+b}{ab}=\frac{7}{43/3}=\boxed{\frac{21}{43}}$.