Suppose independent random samples have been selected from the ???? and ????

populations. Determine the estimated standard error of ????̄ − ????̄ when

(a) ???????? = 50, ???????? = 75, ???????? = 12.4, and ???????? = 16.5.

(b) ???????? = 125, ???????? = 115, ???????? = 88.3, and ???????? = 106.6.

(c) ???????? = 250, ???????? = 239, ???????? = 224.9, and ???????? = 198.1.