I'm upgrading my 6x site to 7x. Would you please be kind enough to provide the D7 version of this simple calculation:

$node_field[0]['value'] = ($node->field_rent[0]['value'] * 12 / 52);

(basically trying to calculate the rent-per-week from the value of rent-per-month)

I've been banding my head against the wall over this for a week now, but nothing seems to work in D7.