The twist of a cotton yarn can be calculated using the formula:
\[
\text{Twist} = \text{Twist Multiplier} \times \sqrt{\text{Ne count}}.
\]
Given:
\[
\text{Twist Multiplier} = 3.5 \, \text{inch}^{-1}.\text{Ne}^{-0.5}, \text{Ne count} = 36.
\]
Substitute the values:
\[
\text{Twist} = 3.5 \times \sqrt{36} = 3.5 \times 6 = 21.
\]
Thus, the twist is \( 21 \) turns per inch.