During July in​ Jacksonville, Florida, it is not uncommon to have afternoon thunderstorms. On​ average, 10.3 days have afternoon thunderstorms. What is the probability that a randomly selected day in July will not have a​ thunderstorm?

To find the probability that a randomly selected day in July will not have a thunderstorm in Jacksonville, Florida, we first note that July typically has 31 days.

If, on average, 10.3 days have afternoon thunderstorms, then the number of days without thunderstorms can be calculated as follows:

\[
\text{Number of days without thunderstorms} = \text{Total days in July} - \text{Days with thunderstorms}
\]
\[
\text{Number of days without thunderstorms} = 31 - 10.3 = 20.7
\]

Now, to find the probability that a randomly selected day does not have a thunderstorm, we use the formula for probability:

\[
P(\text{no thunderstorm}) = \frac{\text{Number of days without thunderstorms}}{\text{Total number of days in July}}
\]

Substituting in the values:

\[
P(\text{no thunderstorm}) = \frac{20.7}{31}
\]

Calculating that gives:

\[
P(\text{no thunderstorm}) \approx \frac{20.7}{31} \approx 0.6677
\]

Thus, the probability that a randomly selected day in July will not have a thunderstorm is approximately **0.6677** or **66.77%**.