1/10 -> Does not speak Hindi or English (Let's call this set A)
1/5 ->Does not speak English
3/7 ->Does not speak Hindi
Out of the 1/5th people who does not speak English, some are in set A.
So, the remaining (1/5-1/10)=1/10 people speak only Hindi.
Similarly, Out of 3/7 who does not speak Hindi,some are in set A.
The remaining 3/7-1/10=23/70 people speak only English.
People who speak only one language=1/10+23/70=3/7