Lily plans to spend all of her $31 to buy different types of pens that cost $2,$3and $4 respectively. If she wants to buy at least 1 pen of each type, What is kthe maximum number of pens that she can buy?
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數學=樂趣

Lily 打算花完他的31元去買不同的筆,筆有2元;3元;4元,每種筆都至少必須買1支,請問有幾種可能性?

4*1-3*7-2*3
4*1-3*5-2*6
4*1-3*3-2*9
4*1-3*1-2*12

4*2-3*7-2*1
4*2-3*5-2*4
4*2-3*3-2*7
4*2-3*1-2*10

4*3-3*5-2*2
4*3-3*3-2*5
4*3-3*1-2-8

4*4-3*3-2*3
4*4-3*1-2*6

4*5-3*3-2*1
4*5-3*1-2-4

4*6-3*1-2*2

-表示加 *表示乘

所以有16種

以後遇到這種題目先從大的數假設,以本題來說,先假設4元的筆有1支,在假設2支.3支....................