He uses 1 tablespoon of lime juice per drink so over 30 days that's 1*30 = <<1*30=30>>30 tablespoons of lime juice
He can get 2 tablespoons of lime juice per lime and he will use 30 tablespoons of lime juice so he will need 30/2 = 15 limes
The limes are 3 for $1.00 and he needs 15 limes so that's 15/3 = $5.00
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