I Solve such questions using this technique: - (Initially I convert the questions for 1 machine or 1worker etc.)
If 1 machine works, It produces 2x in 4*3 = 12 days.
So for 2x it takes No. of days = 12
for 4x it takes No. of days = 24.
it finishes in 24 days, then machine required = 1
if it finishes in 3 days, then machine required = 24/3 = 8 machines.
Always works this way.
If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut.