Let the cost of one book be denoted as $x$, and since each pen costs half of a book, the cost of one pen is $\frac{x}{2}$.

Given that S bought 4 pens and 3 books, we can set up the total cost equation:

$4 \times \frac{x}{2} + 3x = 8$

Simplifying the first term:

$2x + 3x = 8$

Now combine like terms:

$5x = 8$

Solve for $x$:

$x = \frac{8}{5} = 1.6$

Now that we know the cost of one book is $1.6$ dollars, the cost of one pen is:

$\frac{x}{2} = \frac{1.6}{2} = 0.8$

Thus, the cost of one pen is $0.80.

Would you like further details or have any questions?

Here are some related questions to extend your understanding:

1. What would be the total cost if S bought 5 pens and 4 books?
2. If each pen cost $0.80, how many pens can S buy with $12?
3. What is the total cost if S bought 6 pens and 2 books?
4. If the cost of each pen increased by 20%, what would the new cost be?
5. How would the total cost change if S received a 10% discount on the books?

Tip: When solving word problems, assigning variables to unknown quantities and translating conditions into equations helps simplify the process.