How long would it take me to gain 20 pounds?

OK so I'm 6'1, 155 pounds and fairly cut. But I want to gain about 20 pounds, doesn't have to be all muscle, maybe 10 pounds of fat and 10 pounds of muscle would be acceptable. I just want to be heavier. Any tips on accomplishing this? Best weight gain foods?

Also is a split workout good for this? Or should I stick to full body workouts?
How long would it take me to gain 20 pounds?
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