[3/4 - 120 bpm] means that 120 beats = 60 second, but we need to figure out the duration of 3 beats (1 bar) first, which is (60/120) x 3 = 1.5 second. Now if one bar in the new song (4 beats) fits within 1.5 second, each of the beat should have 1.5/4 second in duration. It would need X beats for one minute: More @Wikipedia
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