The average score of Xiaohua's previous math tests was 80 points, and this time he scored 100 points, which just raised the average score of these several times to 85 points. How many times is this his first quiz?
小华的前几次数学测验的平均成绩是80分，这一次得了100分，正好把这几次的平均成绩提高到85分．这一次是他的第几次测验？
<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<gadget id="calculator">85 - 80</gadget>
<output>5</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<result>4</result>