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This problem trains for: SAT-I, AMC-8, GMAT.

In the process of making onion flakes, onions lose 85% of their weight by dehydration. How many lbs of fresh onions are needed to produce 3 lbs of onion flakes?

Denote the amount of fresh onions by a. After dehydration only 15% of the weight is left. This weight must equal 3 lbs:

Therefore, 20 lbs of fresh onions are needed.