You can try here, i.e., GMAT Club DS forum. The best way to start would be to familiarize with the concepts by reading the MATH Book, and then starting from problems which are tagged as 600 Level, and then advancing onto the bigger/tougher ones.
Also, the instructors/moderators here are very helpful and they can help you with your specific queries.
Formula of Life -> Achievement/Potential = k * Happiness (where k is a constant)
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