Hard data structure and algorithm practice puzzle.
Given a monthly growth rate as a percentage, return how many months until an amount doubles. Start at 1, multiply by (1 + rate / 100) each month, and count months until you reach 2. Growth COMPOUNDS, so at 10 percent a month it doubles in 8 months, not 10.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Trade Stats Roll-up, a pipeline you can run and download. See every Software Engineer project.
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