For this case what you should do is the following system of equations:
x: skilled laborers
y: unskilled laborers
By writing the system we have:
x + y = 24
25x + 12y = 405
Solving the system:
Step 1:
-25x + -25y = -600
25x + 12y = 405
Step 2:
-13y = -195
y = 195/13
y = 15
step 3:
x = 24-y
x = 24-15
x = 9
Answer:
skilled laborers = 9
unskilled laborers = 15