中一數學既問題(20分)勁急呀

設通話時間係多過200分鐘, 額外時間為"x"
計劃A電話費係: 基本月費$88 + 100x$0.8 + $0.8x
計劃B電話費係: 基本月費$128 + $1.0x

由於電話費係一樣,所以
88 + 80 + 0.8x = 128 + x
40 = 0.2x
x = 200
(a)通話時間係200+200=400分鐘
(b)電話費係128+200 = $328

Let x=total min used, we can setup an equation base on the equal min used:
Discuss in 3 cases:
1) when x is less than or equal to 100 : Impossible. $88 is not equal to $128.

2) when x is greater than 100 but less than 200:
0.8*(x-100)+88=128 (we got x=150. This satisfied our assumption that 100<200).

3)When x is greater than or equal to 200:
0.8*(x-100)+88=128+1*(x-200) (we got x=400. This satisfied our assumption that x greater than or equal to 200).

Done! :-)

a)小明用左1600mins,小白用左2200mins。
b)小明既電話費係$1420.8,小白既電話費係$1419。