Just another way:
<table border="1" cellspacing="0" style="font-family:Calibri,Arial; font-size:11pt; background-color:#ffffff; "> <colgroup><col style="font-weight:bold; width:30px; " /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /><col style="width:34.22px;" /></colgroup><tr style="background-color:#cacaca; text-align:center; font-weight:bold; font-size:8pt; "><td > </td><td >A</td><td >B</td><td >C</td><td >D</td><td >E</td><td >F</td><td >G</td><td >H</td><td >I</td></tr><tr style="height:19px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >20</td><td style="text-align:right; ">1</td><td style="text-align:right; ">2</td><td style="text-align:right; ">3</td><td style="text-align:right; ">4</td><td style="text-align:right; ">5</td><td style="text-align:right; ">6</td><td style="text-align:right; ">7</td><td style="text-align:right; ">8</td><td style="text-align:right; ">9</td></tr><tr style="height:19px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >21</td><td style="text-align:right; ">6</td><td style="text-align:right; ">15</td><td style="text-align:right; ">24</td><td > </td><td > </td><td > </td><td > </td><td > </td><td > </td></tr></table><br /><table style="font-family:Arial; font-size:10pt; border-style: groove ;border-color:#00ff00;background-color:#fffcf9; color:#000000; "><tr><td ><b></b></td></tr><tr><td ><table border = "1" cellspacing="0" cellpadding="2" style="font-family:Arial; font-size:9pt;"><tr style="background-color:#cacaca; font-size:10pt;"><td >Cell</td><td >Formula</td></tr><tr><td >A21</td><td >=SUM(OFFSET(INDEX(Sheet1!$A$20:$I$20,,(COLUMNS($A$1:A1)-1)*3+1),,,,3))</td></tr></table></td></tr></table>