Let \( s \) represent the speed in miles per hour. Since Nio and her mom expect to arrive at the book fair in 3 hours, we can find the total distance they will travel at that speed using the formula for distance:
\[ \text{Distance} = \text{Speed} \times \text{Time} \]
In this case, they will travel for 3 hours at a speed of \( s \) miles per hour. Therefore, the distance they would cover in those 3 hours is:
\[ 3s \]
Nio has already covered 42 miles so far. Thus, the total distance from Nancy's house to the book fair can be expressed as the distance already covered plus the distance remaining. Therefore, the expression representing the total distance from Nancy's house to the book fair is:
\[ 42 + 3s \]
So, the total distance is \( 42 + 3s \) miles.