part A:
company a
30m+15
m= matress
company b
28m+21
m=matresses sold
part B:
company a
30(6)+15
195$
company b
28(6)+21
189$
company B charges less
part C:
company a
30(3)+15
105$
company b
28(3)+21
105$
there is no change at all so you do not save anything.