By walking at $\frac{5}{4}$ of his usual speed, a man reaches his destination $20 \text{ minutes}$ earlier than his usual time. Find his usual time.

Options:

• A: $80 \text{ minutes}$
• B: $90 \text{ minutes}$
• C: $100 \text{ minutes}$
• D: $120 \text{ minutes}$

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