One week from now, the amount will be ...
... $50 + 0.20×$50 = $50 + 10 = $60
Two weeks from now, the amount will be ...
... $60 + 0.20×$60 = $60 + 12 = $72
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The first expression can be factored to be $50 × (1 + 0.20) = $50×1.2.
The second expression can be factored the same way to be $60×1.2, or ...
... ($50×1.2)×1.2 = $50×1.2²
... = $50×1.44 = $72