Let x = Craig's time
Let y = Tom's time
Create a system of equations to solve this problem.
8 = x + y
y = 4x
Substitute 4x into the y variable in the first equation.
8 = x + 4x. Combine like terms
8 = 5x. Divide each side by 5.
1.6 = x
Craig takes 1.6 hours to clean the house