This could be handled with a formula, I believe:
<b>Excel 2007</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><colgroup><col width="25px" style="background-color: #E0E0F0" /><col /><col /></colgroup><thead><tr style=" background-color: #E0E0F0;text-align: center;color: #161120"><th></th><th>A</th><th>B</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">1</td><td style=";">1-2</td><td style="text-align: right;;">1.5</td></tr><tr ><td style="color: #161120;text-align: center;">2</td><td style=";">1-3</td><td style="text-align: right;;">2</td></tr><tr ><td style="color: #161120;text-align: center;">3</td><td style="text-align: right;;"></td><td style=";"></td></tr><tr ><td style="color: #161120;text-align: center;">4</td><td style="text-align: right;;"></td><td style=";"></td></tr><tr ><td style="color: #161120;text-align: center;">5</td><td style=";">1-4</td><td style="text-align: right;;">2.5</td></tr><tr ><td style="color: #161120;text-align: center;">6</td><td style=";">2-3</td><td style="text-align: right;;">2.5</td></tr><tr ><td style="color: #161120;text-align: center;">7</td><td style=";">2-4</td><td style="text-align: right;;">3</td></tr><tr ><td style="color: #161120;text-align: center;">8</td><td style=";">3-4</td><td style="text-align: right;;">3.5</td></tr></tbody></table><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style="background-color: #E0E0F0;text-align: center;color: #161120"><th><b>Sheet1</b></th></tr></td></thead></table><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style=" background-color: #E0E0F0;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">B1</th><td style="text-align:left">=IF(<font color="Blue">ISERROR(<font color="Red">FIND(<font color="Green">"-",A1</font>)</font>),"",AVERAGE(<font color="Red">LEFT(<font color="Green">A1,FIND(<font color="Purple">"-",A1</font>)-1</font>),RIGHT(<font color="Green">A1,LEN(<font color="Purple">A1</font>)-FIND(<font color="Purple">"-",A1</font>)</font>)</font>)</font>)</td></tr></tbody></table></td></tr></table><br />