这道题我是随便找了个数字代入法算出答案Ｂ的　　如果列出式子2x+1.5y+z=15 如何求最大值呢？／？谢谢

An equal number of desks and bookcases are to be placed

along a library wall that is 15 meters long. Each desk is

2 meters long, and each bookshelf is 1.5 meters long. If

the maximum possible number of desks and bookcases

are to be placed along the wall, then the space along the

wall that is left over will be how many meters long?

A.0.5

B.1

C.1.5

D.2

E.3

设书桌有a张，书架为a个，2a＋1.5a<=15

a为4时，所占据墙的长度为最大，剩下的墙的长度为1米

many thanks !!!!!!!!!