二元一次方程問題，help!!

1) y + 2x = 28 .....(1)
x - y = 1 .........(2)(1) + (2) :3x = 29x = 29/3 , 代入 (2) :29/3 - y = 1y = 26/3
2) x - y = 24 ................(1)
x - 10 = 2(y - 10) ......(2)由 (1) :x = y + 24 , 代入 (2) :y+24 - 10 = 2(y - 10)
y + 14 = 2y - 20y = 34x = 34 + 24 = 58
應用題：設混合米中有甲等米 x kg , 乙等米 y kg :把兩種米混合起來製成30kg的混合米 :x + y = 30 .....(1)每kg的成本是$7.4 :(9x + 5y) / 30 = 7.4 .....(2)由 (1) , x = 30 - y , 代入 (2) :(9(30-y) + 5y) / 30 = 7.4
270 - 9y + 5y = 222
4y = 48
y = 12x = 30 - 12 = 18混合米中分別有甲等米 18 kg 和 乙等米 12 kg。