A, B, and C have a total of 90 extra-curricular books. The number of A's books divided by the number of B's books, and the number of C's books divided by the number of A's books, are all quotients with a remainder of 4. How many books does B have?
甲、乙、丙共有90本课外书，甲的本数除以乙的本数，丙的本数除以甲的本数，都是商4余1．乙有多少本书？
<gadget id="calculator">90 - 1 - 4 - 1</gadget>
<output>84</output>

<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">1 + 4 + 16</gadget>
<output>21</output>

<gadget id="calculator">84 / 21</gadget>
<output>4</output>

<result>4</result>