Answer
n $\approx$ 126
Work Step by Step
1) The z-score corresponding to a 90% Confidence interval is 1.645
2) Substitute z = 1.645, $\sigma$ = 6.8 and E = 1 into the formula:
n = $(\frac{z \times \sigma}{E})^{2}$
n = $(\frac{1.645 \times 6.8 }{1})^{2}$
n $\approx$ 125.13
n $\approx$ 126