Given:
n = 40, sample size
xb = $6.88, sample mean
σ = $1.92, population standard deviation
At 90% confidence interval, the expected range is

= [ 6.88 - 1.645*(1.92/√40), 6.88 + 1.645*(1.92/√40)]
= (6.38, 7.38)
Answer:
The 90% confidence interval is ($6.38, $7.38)