The average number of the first 3 shots is 5 rings, and the average number of the first 4 shots is not less than 6 rings, so how many rings should be shot in the 4th time at least.
前3次打靶的平均数为5环，前4次平均数不低于6环，则第4次至少应打出多少环．
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">24 - 15</gadget>
<output>9</output>

<result>9</result>