Set:
\(n=\text{number of nickles}\\ q=\text{number of quarters}\)
Now, we can make a system of equations. We have:
\(\begin{align*} q+16&=n\\ 5.90&=0.05n+0.25q \end{align*}\)
I trust you can solve this yourself. If not, here it is even more simplified. Substituting \(n\) in, we have:
\(5.90=0.05(q+16)+0.25q\\ 5.90=0.05q+0.8+0.25q\\ 5.90-0.8=0.3q\)
Now finish on your own! :)