Question 1135060
<font color="black" face="times" size="3">
C = price of computer
S = price of speakers
M = price of monitor
all prices are in dollars


one equation is C = 4*S+20 since "A used computer sells for 20 dollars more than four times the amount of a set of speakers"


another equation is M = 3*S-40 because "A used monitor sells for forty dollars less than three times what the speakers sell for"


Finally, the last equation is C+S+M = 580 because the problem states "they sell for 580 dollars all together"


--------------------------------
Use substitution to solve for S
C+S+M = 580 ... start with the third equation
4S+20+S+M = 580 .... replace C with 4S+20
4S+20+S+3S-40 = 580 .... replace M with 3S-40; note how we have one variable now
8S-20 = 580
8S-20+20 = 580+20 ... add 20 to both sides
8S = 600
8S/8 = 600/8 ... divide both sides by 8
S = 75
The speakers sell for 75 dollars


Use this value of S to find M
M = 3*S-40
M = 3*75-40
M = 225-40
M = 185
The monitor is 185 dollars


Finally, use the value of S to find the value of C
C = 4*S + 20
C = 4*75 + 20
C = 300 + 20
C = 320
--------------------------------
--------------------------------
<font color=red>Answers: 
Computer = 320 dollars
Speakers = 75 dollars
Monitor = 185 dollars
</font>
</font>