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.

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If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut.

Albert Einstein