Let the unit prices of Party A, Party B and Party C be X, Y, Z, Y and Z respectively.

So:

x+2y+3z=3 15，3x+2y+z=285，

4(x+y+z)=600

∴x+y+z= 150

Analysis: The difficulty of this problem lies in the correct thinking direction. In fact, the direction is right, and it will be much more convenient to do.

The second question:

6x？ +9y+8

=3(2x？ +3y+7)- 13

=3×8- 13

= 1 1

Analysis:

This question is relatively simple. ∫2x？ +3y+7=8

∴ 2x？ +3y= 1

So: 3(2x? )+3(3y)+8

= 1×3+8= 1 1

Step by step, you must be right.

The third question:

The original formula =-x-[2y-2 *1+3x]+2 [x+4] =-(2x+2y)+10.

∫x+y =(xy+x)-(xy-y)= 1

∴ Original formula =-2+ 10=8

The fourth question:

Electricity 1=a,

Water fee 1=2a+40,

Total 1=3a+40

Electricity fee 2=0.85a,

Water fee 2=2.76a+55.2,

Total 2=3.6 1a+55.2

Total 2- total1= 0.61a+15.2.

Overexpenditure (0.6 1a+ 15.2) yuan.

Analysis: It's actually quite simple to think step by step and do it step by step.

Hehe, I'm also a junior one student. Nice

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You! Let's cheer together. Ask me any questions about math next time. I'm quite sure about math. Ha ha laugh