在一密闭容器中，有甲、乙、丙、丁四种物质，在一定的条件下，充分反应，测得反应前后各物质质量如下表： <table style="border: medium none; border-collapse: collapse; margin-left: 26.4pt;" _dj="MsoTableGrid" border="1" cellpadding="0" cellspacing="0"><tbody><tr style="height: 20.5pt;"><td style="border: 1pt solid windowtext; padding: 0cm 5.4pt; width: 86pt; height: 20.5pt;" width="115">物质</td><td style="border-width: 1pt 1pt 1pt medium; border-style: solid solid solid none; border-color: windowtext windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">甲</td><td style="border-width: 1pt 1pt 1pt medium; border-style: solid solid solid none; border-color: windowtext windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">乙</td><td style="border-width: 1pt 1pt 1pt medium; border-style: solid solid solid none; border-color: windowtext windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">丙</td><td style="border-width: 1pt 1pt 1pt medium; border-style: solid solid solid none; border-color: windowtext windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">丁</td></tr><tr style="height: 20.5pt;"><td style="border-width: medium 1pt 1pt; border-style: none solid solid; border-color: -moz-use-text-color windowtext windowtext; padding: 0cm 5.4pt; width: 86pt; height: 20.5pt;" width="115">反应前质量(g)</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">4</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">1</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">42</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">10</td></tr><tr style="height: 20.5pt;"><td style="border-width: medium 1pt 1pt; border-style: none solid solid; border-color: -moz-use-text-color windowtext windowtext; padding: 0cm 5.4pt; width: 86pt; height: 20.5pt;" width="115">反应后质量(g)</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">待测</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">20</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">6</td><td style="border-width: medium 1pt 1pt medium; border-style: none solid solid none; border-color: -moz-use-text-color windowtext windowtext -moz-use-text-color; padding: 0cm 5.4pt; width: 67.2pt; height: 20.5pt;" width="90">31</td></tr></tbody></table>反应后，甲的“待测”值应为