**A book and 3 pens cost $11.25. The book cost twice as much as a pen. Find the cost of 4 books.**

**Solution**

Book ----- 2 units

Pen ----- 1 unit

1 book + 3 pens ----- $11.25

2 units + 3 units ----- $11.25

5 units ----- $11.25

1 unit ----- $11.25 divided by 5 = $2.25

(4 books) 4 x 2 units ---- 8 units

8 units ----- 8 x $2.25 = $18

**Answer: The cost of 4 books is $18.**