80×200+(80-x)(200+10x)+40[800-200-(200+10x)]-50×800=7000
16000+(16000+600x-10x²)+40[600-200-10x]-40000=7000
16000+16000+600x-10x²+24000-8000-400x-40000-7000=0
1000+200x-10x²=0
x²-20x-100=0
x=10(1±√2)