A part, produced in high volumes, is dimensioned as shown. The machining process making this part is known to be statistically in control based on sampling data. The sampling data shows that $\text{D1}$ follows a normal distribution with a mean of $20 \mathrm{~mm}$ and a standard deviation of $0.3 \mathrm{~mm}$, while $\text{D2}$ follows a normal distribution with a mean of $35 \mathrm{~mm}$ and a standard deviation of $0.4 \mathrm{~mm}$. An inspection of dimension $\mathrm{C}$ is carried out in a sufficiently large number of parts.
To be considered under six-sigma process control, the upper limit of dimension $\mathrm{C}$ should be _____________ $\mathrm{mm}$.
(Rounded off to one decimal place)
