F. if a 2 x 6 (which means the actual dimensions of the board are 1.5 in. x 5.5 in.) milled from the same wood (and therefore has the same density) weighs 3.2 lb, how long is the board?

By definition, we have to W = V * Dw
 W: weight
 V: Volume
 D: Density
 Clearing and Substituting we have:
 V = W / Dw = (3.2 lb) / (0.021 (lb / in ^ 3)) = 152.38 in ^ 3
 Then, on the other hand, the volume is
 V = l * w * d
 l: long w: width
 d: depth
 Clearing and substituting
 l = V / (w * d) = (152.38 in ^ 3) / ((1.5 in) * (5.5 in)) = 18.5 in
 The answer is 18.5 in