Solve the Expression: Finding the Value of 21-4

To solve this problem, we’ll follow these steps:

• Step 1: Calculate the original expression $21 - 4$.
• Step 2: Compare this result with each of the provided choices to find an equivalent expression.

Now, let's work through each step:
Step 1: The original expression $21 - 4$ simplifies directly to $17$.
Step 2: Check each choice:
- Choice 1: $21 - 4 - 1 = 16$ (not equivalent)
- Choice 2: $21 - 3 - 3 = 15$ (not equivalent)
- Choice 3: $21 - 2 - 1 = 18$ (not equivalent)
- Choice 4: $21 - 1 - 3 = 17$ (equivalent)

Therefore, the expression equivalent to $21 - 4$ is $21 - 1 - 3$, which can also be written as $21 - 1 - 3$.