You should use on_change method of OpenERP,

for example,

in your .py file,

'number': fields.integer('Number'),
'sum': fields.integer('Sum'),
'mulitply': fields.integer('Multiply'),

    def onchange_number(self, cr, uid, ids, number,context=None):        
        val = { 'sum': 0, 'multiply': 0}
        if number:
                val['sum'] = number + 4
                val['multiply'] = number * 20
        return {'value': val}

in xml,

<field name="number" on_change="onchange_number(number)"/>
<field name="sum" />
<field name="multiply"/>

Hi,

Use "Function" type field and write fields in store on which it depend.

Thanks
www.acespritech.com