A company wants to compare the efficiency of two types of fuel for a car. 5 identical cars will be driven 500 kilometers each, two with the first type of fuel and three with the second type. Let X₁ and X₂ be the observed fuel efficiency for the first type and Y₁ , Y₂, and Y₃ be the efficiency for the second type. Suppose these variables are independent and Xᵢ ~ N(20,4) for i = 1, 2 and Yⱼ ~ N(18,9) for j = 1, 2, 3. Define a new random variable: W = (X₁ + X₂)/2 - (Y₁ +Y₂ + Y₃)/3. Find E(W) .