In a survey, 31 people were asked how much they spent on their child's last birthday gift. The results were roughly bell-shaped with a mean of $39 and standard deviation of $9. Construct a confidence interval at a 90% confidence level. Give your answers to one decimal place.

u = (41.659, 36.341)

Step-by-step explanation:

Constructing a 90% confidence interval for mean is given by the formulae below.

u = x + Zα/2 * σ/√n This is the upper limit

u = x - Zα/2 * σ/√n This is the lower limit.

u = population mean

x = sample mean = 39

σ = population standard deviation = 9

Zα/2 = Z score for a two tailed test with an a level of significance = 1.645 ( for 90% confidence level)

Upper limit

u = 39 + 1.645 * (9/√31)

u = 39 + 1.645 * (1.616)

u = 39 + 2.6590

u = 41.659

Lower limit

u = 39 - 1.645 * (9/√31)

u = 39 - 1.645 * (1.616)

u = 39 - 2.6590

u = 36.341

3

step-by-step explanation:

in standard form:

a= dictionaries

b= maps

you have to buy 3 dictionaries, so put a 3 next to the a (dictionaries) so that you can multiply them by each other.

a3+b= 60 (the amount of money you have)

we already know the cost of a=15 so replace a with $15

($15 x 3)+b=60

multiply 15 and 3 to get $45 and replace it with $45

45+b=60

and then replace b with 15 to get the total:

45+15=60

that means you have $15 left after buying 3 dictionaries.

15 divided by the cost of a map ($5) equals 3.

so you can buy 3 maps.

sorry if it was confusing!