The burning rates of two different solid-fuel propellants used in air crew escape systems are being studied. It is known that both propellants have approximately the same standard deviation of burning rate; that is ₁ = ₂ = 3 centimeters per second. Two random samples of n₁ = 20 and n₂ = 20 specimens are tested; the sample mean burning rates are x₁ = 18 centimeters per second and x₂ = 24 centimeters per second. Use a = 0.10. Construct a 90% confidence interval on the difference in means.