(a)

h	1	0.5	0.01	0.001
q(120+h)q(120) h	63.39	63.68	63.97	63.97

(b) q(120) 6,398 pairs per month; q'(120) 64 pairs per month per $1 increase in the price.

Therefore, when the price was set at $120 per pair, Sammy Sporting Authority sold 6,398 pairs per month. Further, the demand was decreasing at a rate of 64 pairs per month, per $1 increase in the price.