Cement is needed for the overhaul of the school. It is known that 12 vehicles were transported for the first time, with an average of 4.5 tons per vehicle; for the second time, 9 vehicles were transported, with an average of 4 tons per vehicle. After the two shipments of cement have been used on the construction site for 30 days, there are still 15 tons left. How many tons of cement are used on the construction site per day on average?
学校大修需要用水泥，已知第一次运了12车，平均每车运4.5吨；第二次运9车，平均每车运4吨．两次运来的水泥在工地上用了30天后，还剩下15吨，求工地上平均每天用掉水泥多少吨？
<gadget id="calculator">4.5 * 12</gadget>
<output>54</output>

<gadget id="calculator">4 * 9</gadget>
<output>36</output>

<gadget id="calculator">36 - 15</gadget>
<output>21</output>

<gadget id="calculator">54 + 21</gadget>
<output>75</output>

<gadget id="calculator">75 / 30</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>