Managers of an industrial plant want to determine which of two types of fued, gas or electic, is more cost efficient (measured in cost per unit of energy). Independent random samples were taken of plants using electricity and plants using gas. These samples consisted of 15 plants using electricity, which had a mean cost per unit of $58.1 and standard deviation of $8.18, and 13 plants using gas, which had a mean of $49.1 and standard devlation of $8.42. Assume that the populations of costs per unit are normally distributed for each type of fuel, and assume that the variances of these populations are equal. Construct a 90% confidence interval for the dfference u,, between the mean cost per unit for plants using electricity, , and the mean cost per unit for plants usng gas, . Then complete the table below. Carry your intermediate computations to at least three deamal places. Round your responses to at least two decimal places. (1f necessary, consult a tist of tormulas.) What is the lower limit of the 90% confidence interval What is the upper limit of the 90% confidence lienterval?