The Local Phone Company Offers the Following Plan $\$ 3$ For the First Call of the Day Plus

The local phone company offers the following plan: $\$ 3$ for the first call of the day plus $\$ 0.02$ for each minute OR \$21 per week for unlimited calls. I only make phone calls on Sunday, Thursday and Friday and talk for the same number of minutes on each day. My only concern
is saving money. How many minutes must I spend on the phone each week in order for the flat rate to be the optimal choice?