Let ₁ and ₂ denote true average tread lives for two competing brands of size P205/65R15 radial tires.

(a) Test

H₀: ₁ = ₂

H_a: ₁ = = ₂

at level 0.05 using the following data: m = 45, x = 42500, s₁ = 2200, n = 45, y = 40400 , and s₂ = 1900.

(b) Construct a 95% CI for _{1 2}.