Answer:
$2
Explanation:
Represent the cost of a single photo album by a and the cost of a single pen by p. Then:
4a + 16p = $72, and a = 5p. Substituting 5p for a in 4a + 16p = $72, we get:
4(5p) + 16p = $72, or 16p = 36p = $72. Thus, p = $2.
A pen costs $2.