Solution:

1. Relative Speed (Same Direction):
\[
\text{Speed of Train} = 15 \, \text{m/s}
\]
\[
\text{Time to Overtake} = 10 \, \text{s}
\]
\[
\text{Distance} = \text{Length of Train} = 100 \, \text{m}
\]
\[
\text{Relative Speed} = \frac{\text{Distance}}{\text{Time}} = \frac{100}{10} = 10 \, \text{m/s}
\]
\[
\text{Speed of Man} = 15 - 10 = 5 \, \text{m/s}
\]

2. Relative Speed (Opposite Direction):
\[
\text{Relative Speed} = 15 + 5 = 20 \, \text{m/s}
\]

3. Time to Cross Man (Opposite Direction):
\[
\text{Time} = \frac{\text{Distance}}{\text{Relative Speed}} = \frac{100}{20} = 5 \, \text{s}
\]

Thus, the train will take 5 seconds to cross the man if he is running in the opposite direction.