The ratio of flowers to herbs is 6 : 2. He started with 80 plant. He wants to increase the number of herbs in his garden so that 40% of his plants are herbs. How many more herb plants must he get?
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Since there are 80 plants to begin with, he has 60 flowers and 20 herbs. If he wants herbs to be 40%, then flowers must become 60%. The number of flowers won't change, so 60 flowers is 60% of the total, which means there must be 40 herbs total. So he adds 40-20 = 20 more herbs.