maths(right,very esay 4题- -今年小明的岁数和爸爸、爷爷的岁数之比为2:7:11,又知道小明的岁数与爸爸岁数之和比爷爷的岁数还小12岁,问:今年爷爷几岁?甲、乙两人从A地出发,甲的速度是10千米/小
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maths(right,very esay 4题- -
今年小明的岁数和爸爸、爷爷的岁数之比为2:7:11,又知道小明的岁数与爸爸岁数之和比爷爷的岁数还小12岁,问:今年爷爷几岁?
甲、乙两人从A地出发,甲的速度是10千米/小时,先出发2小时后,乙出发,乙的速度是15千米/小时,问:几小时后乙才能追上甲?
一家商店将某种服装按成本提高50%后标价,又以标价的9折卖出,结果每件衣服乃可获利70元,问这种衣服的成本价.
解{x+y=40.(2)
{4x+2y=110.(2)
在(2)中把其中的一个未知数x用含有y的式子来表示:x=(),再把它代入(3)
它的解是{x=()
{y=()
小明的岁数和爸爸、爷爷的岁数之比为2:7:11,
设小明2x岁,爸爸7x岁,爷爷11x岁
所以2x+7x=11x-12
11x-2x-7x=12
2x=12
x=6
11x=66
所以爷爷66岁
甲2小时走10*2=20千米
设x小时追上
则甲走10x千米,乙15x千米
追上则乙比甲多20千米
15x-10x=20
5x=20
x=4
4小时追上
设成本x
则标价x(1+50%)
打折,x(1+50%)*0.9
所以x(1+50%)*0.9-x=70
1.35x-x=70
0.35x=70
x=200
所以成本是200元
x+y=40 (2)
4x+2y=110(3)
x=40-y
所以4(40-y)+2y=110
160-4y+2y=110
2y=50
y=25
x=15
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