Answer:
A + B = 24
16A + 20B = 434
Explanation:
To write a system of equations for this scenario, let's say that A represents the number of hours machine A ran, and B represents the number of hours machine B ran. 
The first equation will be: 
A + B = 24
because the total number of hours ran is 24. 
The second equation will be:
16A + 20B = 434
because the total number of items made is 434. 
A + B = 24
16A + 20B = 434
First solve for one variable, and let's just do A. 
Using the first equation, A + B = 24, A is equal to 24 - B. 
Substitute this value to the second equation.
16 (24 - B) + 20B = 434
384 - 16B + 20B = 434
4B = 50
B = 12.5 
Now use this value of B to find the value of A.
A + 12.5 = 24
A = 11.5
Machine A ran for 11.5 hours, and Machine B ran for 12 hours.