Karen makes $3000 / (3/4) = $<<3000/(3/4)=4000>>4000 per month.
In three months, Karen would make a total of $4000 * 3 = $<<4000*3=12000>>12000.
To make the same amount of money John would need to work $12000 / $3000 = <<12000/3000=4>>4 months.
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