Página principal
    
Código HTML:
Ver original- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"  
- "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
- <html xmlns="http://www.w3.org/1999/xhtml"> 
- <meta http-equiv="content-type" content="text/html; charset=utf-8" /> 
- <form name="form1" action="#"> 
- <input type="text" value="" name="suma" /> 
- <button onclick="window.open('calc.html','nv','width=400,height=400');">- calcular </button>
 
calc.html    
Código HTML:
Ver original- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"  
- "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
- <html xmlns="http://www.w3.org/1999/xhtml"> 
- <meta http-equiv="content-type" content="text/html; charset=utf-8" /> 
-   
- <script type="text/javascript"> 
- //<![CDATA[ 
- opener.document.form1.suma.value = '25'; 
- //]]> 
Solo te resta crear en calc.html el form para hacer la suma
Saludos