The fifth grade students of Hongxing Primary School did 99 good deeds for the community in March, three times the number of good deeds of the fourth grade students, and the number of good deeds of the sixth grade students was 15 less than the sum of the fourth and fifth grades. How many good deeds do sixth graders have?
红星小学五年级同学3月份为社区做好事99件，是四年级同学做好事件数的3倍，六年级同学做好事的件数比四、五年级的总和少15件．六年级同学做好事多少件？
<gadget id="calculator">99 / 3</gadget>
<output>33</output>

<gadget id="calculator">33 - 15</gadget>
<output>18</output>

<gadget id="calculator">99 + 18</gadget>
<output>117</output>

<result>117</result>