To answer this question, we have that the scale factor is 1 / 12, and it says that the model is 1/12 the size of the actual length.
Then, we have that the actual length is 21 feet. The size of the model is:
![(1)/(12)\cdot21ft=1.75ft](https://img.qammunity.org/2023/formulas/mathematics/college/dbmodrivw2z9eowflyy0yht8imaatsx3e2.png)
We need to remember that units are not mentioned in a scale factor.
Now, we need to transform the size of feet into inches. We know that in one foot we have 12 inches, then, we can take this into account to have the missing dimension item (length in inches):
![1.75ft\cdot(12in)/(1ft)=21in](https://img.qammunity.org/2023/formulas/mathematics/college/5gff0oqlvkfv9qr2mpyzkyymzan8u8t22r.png)
Therefore, the missing dimension is 21 inches (the model is 21 inches in length.)