There is a two-digit number, if you subtract 5 from it, the result is a multiple of 4; if you subtract 6 from it, the result is a multiple of 5. What is the smallest two-digit number?
有一个两位数，若将它减去5，则所得结果是4的倍数；若将它减去6，则所得结果是5的倍数．则这个两位数最小是多少？
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<result>21</result>