Answer
Neither the husband nor the wife watched television in $3$ of the $30$-day period.
Refer to the Venn Diagram below,
Work Step by Step
Draw two circles that overlap and label them (Husband, Wife).
The husband and wife watched television together for 18 days so write $18$ in the region where the circles overlap.
Since the husband watched television for a total of $20$ days, then it means that in $20-18=2$ of those days, he watched all by himself.
Since the wife watched television for a total of $25$ days, then it means that in $25-18=7$ of those days, she watched all by herself,
Write $2$ in the non-overlapping region inside the circle for Husband.
Write $7$ in the non-overlapping region inside the circle for Wife.
The two circles contain 18, 2, and 7.
Thus, either of them watched television for a total of $18+2+7=27$ days.
Therefore, in the given 30-day period, neither the husband nor the wife watched television for $30-17=3$ days.
