Answer:
The answer to your question is 12 monitors
Step-by-step explanation:
Data
v = monitors weighing 25 lb
f = monitors weighing 40 lb
Total weight = 680 lb
Total monitors = 20
Process
1.- Write equations
v + f = 20 ------------------ (l)
25v + 40f = 680 -----------------(II)
2.- Solve the system by elimination
Multiply equation l by -25
-25v - 25f = - 500
25v + 40f = 680
0 + 15 f = 180
Solve for f
f = 180 / 15
f = 12
3.- Find the value of v
v + 12 = 20
v = 20 - 12
v = 8
4.- Conclusion
There are 12 monitors weighing forty lb
There are 8 monitors weighing 25 lb
