To express the situation with Skylar's coins as an inequality, we need to consider the total number of coins she has, which consists of \( n \) nickels and \( d \) dimes. According to the problem, the total number of coins is at most 21. This can be written as:
\[ n + d \leq 21 \]
Here:
- \( n \) represents the number of nickels.
- \( d \) represents the number of dimes.
Thus, the inequality \( n + d \leq 21 \) accurately captures the given situation.