Let the total number of students be 100.

• Students who passed in English = $80\%$.
• Students who passed in Hindi = $70\%$.
• Students who failed in both subjects = $15\%$.

Using these, we can find the percentage of students who passed in at least one subject:

• Students passing at least one subject = $100 - 15 = 85\%$.

Now, to find those who failed in only one subject:

• Students failing in only one subject = Total passing at least one subject - Students passing both subjects.

Calculate the overlap:

• Passed in both subjects = $80 + 70 - 85 = 65\%$.

Finally:

• Students failing in only one subject = $85 - 65 = 20\%$.

Answer: (B) $20\%$