(p-10 q)(pq+25)+5(50q-5p)=(p-10 q)(pq+25)+25(10q-p)=(p-10 q)(pq+25)-25(p-10q)=
=(p-10q)(pq+25-25)=pq(p-10q)
(p-10 q)(pq+25)+5(50q-5p)=(p-10 q)(pq+25)+25(10q-p)=(p-10 q)(pq+25)-25(p-10q)=
=(p-10q)(pq+25-25)=pq(p-10q)