找出以下数据的均值：<table class=table table-bordered"><tbody><tr><td>X</td><td>19</td><td>21</td><td>23</td><td>25</td><td>27</td><td>29</td><td>31</td></tr><tr><td>f</td><td>13</td><td>15</td><td>16</td><td>18</td><td>16</td><td>15</td><td>13</td></tr></tbody></table>"