x+y+z=70 ①
x/30+y/20+z/40=2/5 ②
x/40+y/20+z/30=10/23 ③
①-30*②得 -1/2y+1/4z=58 ④
①-40*③得 -y-1/3z=1210/23 ⑤
2*④-⑤得 5/6z=1458/23 z=8748/115
代入z到④,得y=-8966/115
代入y和z到①,得x=8268/115
因此 x=8268/115 y=-8966/115 z=8748/115