(i) To find the standard deviation, use the formula:

\(\text{sd}^2 = \frac{\Sigma(x-c)^2}{n} - \left(\frac{\Sigma(x-c)}{n}\right)^2\)

Substitute the given values:

\(\text{sd}^2 = \frac{1957.5}{30} - \left(\frac{234}{30}\right)^2\)

\(\text{sd}^2 = 65.25 - 6.084\)

\(\text{sd}^2 = 59.166\)

\(\text{sd} = \sqrt{59.166} \approx 2.1\)

(ii) Given the mean is 86, use the formula for the mean:

\(\bar{x} = \frac{\Sigma x}{n} = 86\)

\(86 = \frac{234}{30} + c\)

\(86 = 7.8 + c\)

\(c = 86 - 7.8\)

\(c = 78.2\)