Answer:
Maren's basis for the stocks = 132 stocks x $16 = $2,112
18 months later when she sold her stock at $21 per stock, she received $2,772 (= $21 x 12 socks)
Maren's gain = $2,772 - $2,112 = $660
Maren's gain shouldn't be considered as marginal income since it is a capital gain.
I guess we could assume she has to pay the highest rate = 20% x $660 = $132