[問題] GWD5 -Q3

[kwtluck]

Q3:
Material A costs $3 per kilogram, and material B costs $5 per kilogram. If 10 kilograms of material K consists of x kilograms of material A and y kilograms of material B, is x > y?
(1)y > 4
(2)The cost of the 10 kilograms of material K is less than $40.

Answer: B

請各位大大幫忙解題!!~~Thanks

[spreewell]

(1)Y>4 此條件下有可能是X=Y或X<Y或 X>Y 所以不充分
(2)The cost of the 10 kilograms of material K is less than $40
此條件下，我們可得以下兩個式子：
3X+5Y<40 及X+Y=10 我們將第1式取極限為3X+5Y=40結合第2式，我們可得X=5、Y=5 ，此時為Y最大之情形，所以Xㄧ定大於Y
所以答案為B

[kwtluck]

了解~了解
**謝謝spreewell大大耐心地講解**