(a) Either purchasing the desktop or the notebook would be a positive-NPV project. However, you should purchase the desktop, because it is cheaper (more bang for the buck).

(b) You should still purchase the desktop. The marginal cost is $1,500. The marginal benefit is $11,000 −

$9,000 = $2,000.

(c) You should not purchase the notebook. The marginal cost is $2,500. The marginal benefit is $2,000.