40.64 cm is the answer to your question.
Answer:

Explanation:
From the given information, the LED is operating with a given wavelength of 850 nm or 0.85 μm.
Hence, the material dispersion is 
Now, using the pulse spread formula:


Thus, the pulse spreading as a result of material dispersion is:
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Do you speak a little English cuz I can’t help you if a can’t understand you