To find the number of books when the costs for the two plans are equal, we need to set up the equations based on the cost structures of each plan.

Plan 1:
Total cost $y_1 = 6 + 1x$
This can be simplified to:
$y_1 = 1x + 6$

Plan 2:
Total cost $y_2 = 2.5x$

To find the point where the costs are equal, set the equations equal to each other:
$1x + 6 = 2.5x$

Now, solve for $x$:

1. Rearrange the equation:
   $6 = 2.5x - 1x$
   $6 = 1.5x$

2. Divide both sides by 1.5:
   $x = \frac{6}{1.5} = 4$

Thus, the number of books when the costs for the two plans are equal is $4$.

Answer: B. 4

Would you like any further details or have any questions? Here are some related questions to consider:

1. How would the costs change if Renee bought more than 10 books?
2. What is the cost difference between the two plans for 5 books?
3. At what point do the costs for the two plans start to diverge?
4. If Plan 1 had a higher monthly fee, how would that affect the crossover point?
5. How does the cost of books in Plan 2 compare to the additional cost per book in Plan 1?

Tip: When comparing linear cost functions, finding the intersection point (where costs are equal) is a useful method to determine which plan is more economical based on usage.