When a number is divided by 4, the quotient is 21 with a remainder. What is the maximum number.
一个数除以4，商是21，有余数．这个数最大是多少．
<gadget id="calculator">21 * 4</gadget>
<output>84</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">84 + 3</gadget>
<output>87</output>

<result>87</result>