12) $$8(2m-4n+5c)-2(8m+n-4c)+4(m-2n+8c) = 16m-32n+40c-16m-2n+8c+4m-8n+32c = (16-16+4)m + (-32-2-8)n + (40+8+32)c = 4m-42n+80c$$
Ответ: $$4m-42n+80c$$